on the origin and evolution of major gastropod ... - Oxford Journals

J. Moll. Stud. (1988), 54, 367-441.

© The Malacological Society of London 1988

ON THE ORIGIN AND EVOLUTION OF MAJOR GASTROPOD GROUPS, WITH SPECIAL REFERENCE TO THE STREPTONEURA GERHARD HASZPRUNAR Instimt fur Zoologie der UniuersixSt Innsbruck Technikerstrafie 25, A-6020 Innsbruck, Austria (Received 15 October 1987, Accepted 3 March 1988)

(9) The Caenogastropoda are a holophyletic group, of which the Cerithioidea represent the basic stock. The phytogeny and evolutionary history of the strep- The Stenoglossa (= Neogastropoda) probably oritoneuran Gastropoda is reconsidered in the light of gjned from *Neotaenioglossa* sharing distinct simi(i) recent discoveries of different types of organi- larities in sperm-morphology. (10) Campanile symbolicum Iredale, 1917 (Camzation, (ii) new data sets based on modern techniques, and (iii) the dado-evolutionary method to trace genea- panilimorpha) is a probable representative of the first logical relationships. The phylogenetic analysis by group that made a distinct step towards the euthymeans of traditional (homology-analogy) and cladistic neuran organization. (apomorphy-plesiomorphy) character analysis reveals (11). Valvatoidea (Ectobranchia) form a separate several conclusions: offshoot outside the Caenogastropoda. They appear (1) The Gastropoda originated by torsion from as an independent side-branch at the base of the monoplacophoran ancestors after the Cephalopoda allogastropod grade. (12) The 'AUogastropoda*, a grade, include the split off. Torsion itself is understood as a two-step process, resulting in advantages for the larvae (pre- fossil Ncrineoidea and at least four recent lines (Archisence of operculum) and for the adults (anterior tectonicoidea & Omalogyridae; Rissoelloidea; Glamantle cavity). All Gastropoda form a holophyletic cidorboidea; Pyramidelloidea) which represent a step by step evolution towards the euthyneuran level of group. (2) The proposed gastropod archetype differs organization. (13) Euthyneura are monophyletic. However, the largely from previous suggestions in being more similar to docoglossate gastropods than to zeugobranchs. status of the Opisthobranchia (holo- or paraphylen'c) (3) The Docoglossa are regarded as the earliest is still ambiguous. The Pulmonata (including the Gymgastropod offshoot, having retained ancestral (ste- nomorpha) represent a holophyletic assemblage and the crown group of Gastropoda. reoglossate) radula conditions. (14) The respective phylogram (Fig. 5) is trans(4) Based on various lines of evidence the 'symmetrical' limpet groups (Docoglossa, including hot- formed in a classification by the use of the so-called vent group-C?, Cocculiniformia), primary without dado-evolutionary method which enables an helicoid coiling of teleoconch, are accepted as primi- unequivocal retransformation and expresses the different degrees of likelihood (correlated with the main tive for the Gastropoda. (5) CocculinifOnnia, Neritimorpha and possibly evolutionary gaps) in the proposed phylogenetic pathhot-vent group-A represent distinct archaeogastropod way of the streptoneurous Gastropoda. radiations. (6) The Vetigastropoda, originally including zeugobranchs and trochoids are a holophyletic group and include also the Lepetodriloidea (= hot-vent groupCONTENTS B). In contrast, the Neomphaloidea and Seguenzioidea represent distinct lines of evolution. (7) According to their hypoatnroid OT dystenoid Page nervous system the architaenioglossate groups are 1. INTRODUCTION 369 included in the 'Archaeogastropoda* which are defined as an orthophyletic grade. 2. CHARACTER ANALYSIS 370 (8) A major evolutionary gap (a large increase in size enabled planktotrophic larvae; epiathroid ner- 2.0. General remarks 370 vous system) separates the higher streptoneurans (= 370 •Apogastropoda*) from the *Archaeogastropoda*. 2.1. Shell and operculum 370 2.1.1. Limpets The Loxonematoidea are regarded as the common 372 stem group of Caenogastropoda as well as of the 2.1.2. Operculum ectobranch-allogastropod-euthyneuran line (Hete372 2.1.3. Shell slit robranchia). 372 2.1.4. Shell structure ABSTRACT

368 2.1.5. Shell pores 2.1.6. Larval shells 2.1.7. Size 2.2. Shell muscles 2.3. Mantle and subpallial cavity 2.4. Gills 2.4.1. Gills of Vetigastropoda 2.4.2. Gills of Docoglossa 2.4.3. Gills of Cocculiniformia 2.4.4. Gills of Neritimorpha and Melanodrymia 2.4.5. Monopectinate (= pectinibranch) gills 2.4.6. Ectobranch gills 2.4.7. Foliobranch gills of Allogastropoda 2.4.8. Plicate gills of Euthyneura (= Pentaganglionata) 2.5. Heart, circulatory and excretory system 2.5.1. Heart 2.5.2. Circulatory system 2.5.3. Excretory system 2.6. Genital system and reproduction 2.6.1. Anatomy of genital system 2.6.2. Spermatozoa 2.6.3. Eggs and spawn 2.6.4. Larvae 2.6.5. Development 2.7. Alimentary tract 2.7.1. Radula 2.7.2. Buccal apparatus 2.7.3. Oesophagus 2.7.4. Posterior alimentary tract 2.8. Nervous system 2.8.1. Streptoneury-euthyneury 2.8.2. Hypo- and epiathroid condition 2.8.3. Other characters of the nervous system 2.9. Sense organs 2.9.1. Cephalic tentacles 2.9.2. Epipodium 2.9.3. Eyes 2.9.4. Osphradium 2.9.5. Bursicles and similar structures 2.9.6. Subradular organ 2.9.7. Statocysts 2.10. General conclusions

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3.1.2. Monophyly of Conchifera 3.1.3. Tryblidiida 3.1.4. Diasoma (= Loboconcha, Ancyropoda) 3.1.5. Cyrtosoma (s. str. = Visceroconcha, Rhacopoda) 3.2. Torsion - the key to the Gastropoda 3.2.1. Ontogeny of torsion 3.2.2. Anatomical context of torsion 3.2.3. On the adaptive significance of torsion 3.3. Paleontological problems 3.4. The gastropod archetype

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4. SUBGROUPS OF GASTROPODA

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4.1. The first gastropods - symmetrical limpets 4.1.1. Docoglossa 4.1.2. Hot-vent group C 4.1.3. Cocculiniformia 4.2. The first coiled forms 4.2.1. Neritimorpha 4.2.2. Melqnodrymia and Hot-vent group A 4.3. Vetigastropoda and relatives 4.3.1. Vetigastropoda 4.3.2. Neomphalus 4.3.3. Lepetodriloidea 4.4. Archaeo- or Caenogastropoda? 4.4.1. Seguenziina 4.4.2. Architaenioglossa 4.5. Caenogastropoda, the field of success 4.5.1. Cerithioidea - the basic stock 4.5.2. Ctenoglossa 4.5.3. Neotaenioglossa 4.5.4. Stenoglossa (= Neogastropoda) 4.6. Ectobranchia and relatives 4.6.1. Valvatoidea 4.6.2. Possible relatives 4.7. The allogastropod grade 4.7.1. General remarks 4.7.2. Architectonicoidea and relatives 4.7.3. Omalogyridae 4.7.4. Rissoellidae 4.7.5. Glacidorbidae 4.7.6. Pyramidelloidea and relatives 4.8. Euthyneura (= Pentaganglionata) 4.8.1. Monophyly of Euthyneura

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3. ON THE ORIGIN OF GASTROPODA

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3.1. Gastropoda as Mollusca 3.1.1. The aculiferan groups

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ORIGIN AND EVOLUTION OF GASTROPODS

4.8.2. Distinction between Opisthobranchia and Pulmonata 4.8.3. Origin of the Pulmonata 4.8.4. Subgroups of Euthyneura 4.9. General conclusions

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5. ON CLASSIFICATION

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5.1. On a "natural" system 5.2. A phylogenetic system of the Gastropoda

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ACKNOWLEDGEMENTS

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REFERENCES

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1. INTRODUCTION Since its publication Thiele's (1929) classification of streptoneurans Gastropoda has provided the main framework for phylogenetic discussion. Therefore .aside from minor changes this system has reached a high degree of acceptance (e.g. Wenz, 1938; Taylor & Sohl, 1962; Fretter & Graham, 1962; Boss, 1982). Only a few authors felt the need for a different system of classification for Recent Gastropoda (e.g. Golikov & Starobogatov, 1975). However, Fretter & Graham (1982:364) recently alluded to the inadequacy of the present system: 'the classification of prosobranch gastropods is in need of radical revision if these facts are to be properly reflected'. Three main factors have led, during the last few years, towards a new understanding of gastropod evolution and require the establishment of a new phylogenetic system: The first reason is the discovery of new gastropod groups of high rank and the reinvestigation of species with previously unknown internal organization. Recent work on deepsea faunas, in particular those from the newly discovered habitat of the hydrothermal vents in the East Pacific Ocean, has revealed several new groups of archaeogastropods (McLean, 1981, 1985b, 1988,1989; Fretter et al., 1981; Hickman, 1983,1984a; Fretter, 1988; McLean & Haszprunar, 1987; Haszprunar, 1989). Anatomical examination of many groups, which were previously known only with respect to shell and radula, has significantly improved the character basis for phylogenetic analysis (e.g. Quinn, 1983,1984; Haszprunar, 1985d, f, 1987a, 1988a, b, d, unpubl.; Ponder, 1986, 1988a, b). The second reason is found in new methods, which have been adopted during the last two

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decades. Among these fine-structural investigations (SEM and TEM) are the most important. In particular, ultrastructural results on shell ontogeny and structure (e.g. Bandel, 1979a, 1982), on radular morphology (e.g. Hickman, 1983, 1984a), on sense organs (e.g. Haszprunar, 1985a, b, 1987b), and on sperm morphology (e.g. Kohnert & Storch, 1983, 1984a, b; Koike, 1985; Healy, 1982,1983,1986ad, 1988) provided many new data which are very useful for phylogenetic analysis. Plastic embedding and semi-thin (0.5-3 \tm) serial sectioning is still rare in microanatomical investigations of tiny organisms and larval stages (e.g. Smith & Tyler, 1984). Compared with traditional histological techniques (paraffin/ paraplast or paraffin/celluidin embedding), semi-thin sectioning has advantages in specimen orientation of organisms down to 70 \tm length (personal experience) and also because of much better optical resolution in the much thinner sections. Especially in cases of tiny animals, very hard or mucous tissues, or yolk-rich embryos, semi-thin sectioning will become one of the leading tools in comparative microanatomy at the light microscopical level. It is of particular interest, that semi-thin methods can also be used on very old preserved material. In addition to these methods, new cytological and biochemical techniques have been used to trace systematic relationships. However, in most cases the importance of such methods (chromosome number and -banding, composition of shell matrix, and others) is so far restricted to lowlevel systematic categories, and only trends can be given with respect to higher classifications (e.g. Patterson, 1967; Davis, 1976; Nakamura, 1987). The third reason is a theoretical one. It has become essential in phylogenetics to distinguish between synapomorphic (shared derived) and symplesiomorphic (shared primitive) homologies (e.g. Hennig, 1966; Wiley, 1981; Ax, 1984; Wheeler, 1986). This distinction leads to a much more precise argument in favour of a proposed evolutionary hypothesis. The mode of translation of the proposed final phylogram into a phylogenetic and practical system is heavily debated at present (e.g. Hennig, 1966, 1974; Wiley, 1979,1981; Mayr, 1974, 1981; Ax, 1984) with a possible synthesis being proposed by the author (Haszprunar, 1986). The author has already presented some suggestions towards a new classification of Gastropoda. These were either restricted to certain organs (Haszprunar, 1985a, b, 1987b), or certain levels of evolution (Haszprunar, 1985c)

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or were of preliminary character (Haszprunar, Apomorphic characters can only be understood 1988c; Salvini-Plaw6n & Haszprunar, 1987). by their plesiomorphic counterparts, when they The present analysis is restricted to the higher represent an evolutionary novelty. The latter level of systematics and primarily sets out to case, however, falls under the traditional field clarify the main lines of gastropod evolution. of homology research (see Rieger & Tyler, 1985; Firstly, the different organ systems and charac- Westheide & Rieger, 1987). In addition, pleters are discussed with respect to their phylo- siomorphic characters are necessary to recongenetic value and importance (section 2, Table struct the archetype of a group. However, 2). Based on this character analysis the origin plesiomorphic characters must be differently of the Gastropoda and the position of the class used, because they reflect grades, whereas apoamong the Mollusca is reconsidered (section 3). morphic characters reflect clades. Secondly, it The presentation of the proposed evolutionary is essential to know how often the plesiomorphic line of the gastropods, and the status and inter- stage changed into the apomorphic one (again relationships of the subgroups follows (chapter a problem of traditional homology research), 4). The result of the latter considerations is a and also this can only be done by carefully phylogram of the (streptoneuran) Gastropoda studying both character states. (Fig. 5). Finally, the translation of a phylogram In the following these considerations are exeinto a classification is discussed (section 5). cuted with respect to the organs of the streptoneuran Gastropoda. 2. CHARACTER ANALYSIS 2.1. Shell and operculum 2.0. General remarks As I have done in my former papers, the ending '-oidea' is used for superfamilies, according to the proposition of the International Commission of Zoological Nomenclature. With respect to taxa I use traditional names, if the respective subgroups have not been drastically changed (e.g. Docoglossa, Euthyneura). I use new taxa, however, in the case of major changes, or to express phylogenetic relationships unequivocally (e.g. Vetigastropoda, Apogastropoda). For authors of taxa see Table 5a, b. The following character analysis distinguishes between homology and analogy (•= convergence) as well as between apomorphy and plesiomorphy. For the theoretical background of these distinctions see § 5.1. It is often argued that analogies have no value in a phylogenetic discussion. This is true in that one must not base a phylogenetic relationship on similarities which are the result of convergence. On the other hand, known analogies are very valuable, because they help tracing the evolutionary advantages of a distinct character (-state). Therefore analogies may play an important role in systematics. It should be stressed that the terms apomorphic and plesiomorphic are relative, being used with respect to a particular group. For instance streptoneury is apomorphic for the Gastropoda, but is plesiomorphic for the streptoneurous grade. Again it is often argued that phylogenetics must be solely based on apomorpbies, and again this is simply not true.

The shell is one of the most important characters used to identify gastropod species. However, the use of shell characters in higher systematics is limited, since convergence is known at all levels of classification. 2.1.1. Limpets The limpet shell form is generally accepted as one of the most typical cases of convergence. It seems to be a dogma in gastropod conchology that limpets always must have coiled ancestors (e.g. Yonge, 1947; Eales, 1950; McLean, 1981, 1984). In fact, there is good evidence that the great majority of limpet-like groups are descendants of coiled ancestors. In all these cases a more or less coiled juvenile teleoconch is present, and/or coiled relatives exist. However, there are three rather isolated groups of archaeogastropod limpets, where no trace of a helicoid juvenile teleoconch can be found. This phenomenon has been known for a long time in the case of the Docoglossa (e.g. Morse, 1910; Thompson, 1912; Thorson, 1946; Anderson, 1965; Bandel, 1982), but only Wingstrand (1985) has suggested this condition to be primitive. Lindberg (1988) explains the phenomenon by paedomorphosis, since nearly all archaeogastropods have symmetrically coiled embryonic shells (Bandel, 1982). However, there is no evidence for paedomorphosis in Docoglossa, which is in contrast to the opisthobranch Thecasomata for example, where paedomorphosis of the shell is suggested,


because adult shell structure is identical to those of larval shells of other opisthobranch groups (Be et al., 1972; Rampal, 1973; Richter, 1976). Furthermore, the adult shells of primitive thecasomes are hyperstrophic, as are larval opisthobranch shells in general (Haszprunar, 1985c). Aside from the apparent lack of evidence, it is hard to accept that paedomorphosis occurred in three independent offshoots. The second group of 'symmetrical' limpets are the Cocculiniformia (Haszprunar, 1988d), and the special conditions in certain subgroups might help to clear up the problem. In the majority of families no trace of juvenile helicoid coiling of the teleoconch exists (e.g. Moskalev, 1978; Bandel, 1982; Marshall, 1983, 1986). In the Addisoniidae, which are highly derived because of their alimentary tract and gill-type, the juvenile teleoconch is perfectly symmetrical, whereas the adult shell becomes asymmetrical (McLean, 1985a), a unique situation among gastropods. This condition has previously been explained by the hypertrophied addisoniid gill in the right subpallial cavity. However, there are other cocculiniform groups with similar gills (Pyropeltidae, certain Pseudococculinidae, Osteopeltidae) but having symmetrical shells (Marshall, 1986,1987; Haszprunar, 1988a, b, d; McLean & Haszprunar, 1987), thus weakening this argument. Moreover, the Choristellidae, which are closely related to the Addisoniidae and are therefore also highly derived, include normally coiled forms (Haszprunar, 1988d). In summary, the conditions in the CoccuUniformia suggest both primary limpets and derived coiled forms in this group. A third group of 'symmetrical' limpets has recently been discovered from hydrothermal vents of the East Pacific Rise (group C: McLean, 1985b). Up to now the anatomy of these forms (four species are known) is poorly known, but resembles that of the Docoglossa (see §4.1.) as does the shell, whereas the radula is unique (Hickman, 1983). Summing up so far, it is possible that the limpet shell in the above-mentioned groups is a primitive character. This possibility was mentioned by Bandel (1982:142): 'One can imagine torted molluscs, the shell of which did not become secondarily bilateral-symmetrical as in many gastropods of the recent fauna, but which were primarily bilateral-symmetrical' (English translation of the original German). This hypothesis will be reinforced by evidence from different sources. Based on his detailed anatomical studies of

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Recent Neopilinidae Wingstrand (1985) stressed the similarities between the Neopilinidae and the Docoglossa (see also § 4.1.). Although different in structure, the shells of both groups are so similar (symmetrical limpets with anterior apex) that Neopilina zographi (Dautzenberg & Fischer, 1897) was originally described as an Acmaea (Bouchet et al., 1983). Even Odhner, who created the concept of Monoplacophora, labeled a Neopilina sample as Acmaea (Waren, 1987). In addition, the neopilinid radula is nearly identical in shape and function to those of certain Lepetidae (Moskalev, 1977; McLean, 1979; Wingstrand, 1985; Salvini-Plawen, 1988a). Thus, outgroup comparison with the Recent Tryblidiida makes it likely that the limpet shell of the Docoglossa is a primitive character. Contrary to earlier suggestions, there is no primary correlation between gastropod torsion and helicoid form of the teleoconch. Evidence for this is two fold: (i) In ontogeny, coiled archaeogastropods have the embryonic shell nearly symmetrical after the torsion process, the helicoid phase starts much later (Bandel, 1982). (ii) In the Allogastropoda (see §4.7.1.) and primitive Euthyneura the larval shell is hyperstrophic, and spontaneous mutations from orthostrophic to hyperstrophic teleoconchs are known (Robertson & Merrill, 1963). This demonstrates that direction of shell coiling is not correlated with the direction of torsion. Paleontological support of the idea of primary limpet shells comes from two sources. (1) It has become largely accepted by paleontologists during recent years that asymmetrical shell coiling occurred several times among early Conchifera (Bandel, 1982; Harper & Rollins, 1982; Runnegar & Pojeta, 1985). Linsley & Kier (1984) went so far to propose a new (probably polyphyletic) class Paragastropoda for such forms. Assuming that torsion is independent from teleoconch coiling, there is no reason to reject multiple evolution of coiling in torted gastropods. In any case, in the light of limpetlike, ortho- or hyperstrophic fossils, which may belong to torted or untorted organisms, the situation has become confusing for paleontologists. (2) On the base of preserved opercula there are certain hyperstrophic gastropods (Macluritoidea) known from the Paleozoic (Rohr & PotteT, 1987), demonstrating again that torsion and orientation of teleoconchs are not directly correlated. (3) The Bellerophontida might have originated from a primary (torted) limpet by evolving planispiral shells and bodies (see below) parallel to the rest of gastropods.

372 G. HASZPRUNAR This would explain the lack of opercula among thus a greater possibility for specialization), the Bellerophontida as well as the various pat- greater mobility of the animal (Linsley, 1978), terns of muscle scars. and the utilization of the larval operculum to In the Gastropoda the path from limpet to close the shell and thus to protect the body. The coiled shell possibly occurred in two steps. The last advantage postulates a paedomorphosis of Neritimorpha are thought to be the first offshoot the (primary) larval operculum. Paedomorof coiled gastropods (see § 4.2.1.). In this group phosis is also known in the Bivalvia, where the there is a unique mode of shell development byssus gland is paedomorphically retained in (Bandel, 1982), and the adult shell lacks a true adults of many groups (Yonge, 1962). columella, resulting in a 'coiled limpet' (Thompson, 1980). Thus, the Neritimorpha may represent an intermediate type between primary 2.1.3. Shell slit symmetrical limpets and 'true' coiled forms with For a long time the shell slit of certain archaeoa columella. gastropods has been accepted as a good character used to trace phylogenetic relationships. Recently, however, Bandel (1982: 45) weake2.1.2. Opcrculum ned the argument, stating that a shell slit occurs The existence of an operculum in larval Doco- in very different groups. Among the Vetiglossa cannot be doubted (e.g. Smith, 1935; gastropoda the shell slit clearly is a pleThorson, 1946; Crofts, 1955; Anderson, 1965), siomorphic character, since the Trochoidea are and it is also present in juvenile Lepetellidae more closely related to the Pleurotomarioidea (War6n, pers. comm.). If one accepts the idea than are the Fissurelloidea (see §4.3.1.). The of primary limpets in gastropods, why do the occurrence of a posteriorly placed slit in the larvae of (primary) limpets need an operculum? polyplacophoran Schizochiton weakens the In contrast to shell coiling, the coiling direction arguments in favour of a gastropod (torted) of gastropod opercula is directly correlated with nature of the Bellerophontida, because a counthe direction of torsion. This can be demon- ter-example is thus present. Furthermore, higher strated in cases of hyperstrophic shells where gastropods such as the Siliquariidae (Gould, the operculum shows a correctly orientated 1966) and certain Atlantidae, Turritellidae and spiral (i.e. counter-clockwise in dextral forms, Turridae possess a shell slit, which in these cases e.g. Bieler, 1984a: fig. 5). This correlation also is obviously a phenomenon of convergence. proves the spiral operculum (as organ) to be a Thus the shell slit, used alone, is of little phylotypical gastropod structure. The operculum is genetic importance. found during ontogeny in all gastropods with free larval development, whether they become limpets, helicoids, aberrant forms, or slugs. Contrary to earlier results (e.g. Underwood, 2.1.4. Shell structure 1972), Bandel (1982) reported that the archaeo- Shell structure is one of the main characters for gastropod larva can be fully retracted into the determining and classifying fossil gastropods. shell. He (1982: 141) emphasized the import- Many groups are characterized by the distinctive ance of the operculum in protecting the soft structure of their shell and its value for lower body of the larva in the contracted condition. systematics (e.g. Docoglossa, see MacClintock, Thus, it is likely that the gastropod operculum 1967; Lindberg, 1988) is without doubt very was primarily a larval character, being lost du- high. However, shell structure is a very ambiguring settlement in primary limpets. ous character in higher systematics. Firstly, To state that the operculum was originally a there are many cases of convergence (e.g. the larval character also enables speculation on the transition from cross lamellar to prismatic evolutionary advantage of coiled forms. The layers, see Bandel, 1979a). In addition, there number of independently evolved (Linsley & are several groups such as the Trochoidea, Kier, 1984) asymmetrically coiled fossils, be where great variation in shell structure contrasts they torted or not, show that it has always been with a rather uniform anatomical groundplan. a great advantage to evolve helicoid shells. Also, The occurrence of nacre is clearly a pleamong Recent Gastropoda, coiled shells have siomorphic character and is restricted to the been evolved at least twice (Coccuuniformia- archaeogastropod grade, but there are several Choristellidae, higher Gastropoda). The obvi- archaeogastropod groups where nacre is ous advantages of coiling are a higher degree of lacking. Finally, in the investigation of fossils the shell stability, more freedom of shell form (and danger of artefacts caused by remineralization


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is considerable, especially in crucial Paleozoic fossils or empty shells without knowing the anatomy of the organisms. On the other hand forms (e.g. Harper & Rollins, 1982). Neritimorpha and Pelycidiidae have very characteristic teleoconchs. Parallel conditions 2.1.5. Shell pores exist in the Bivalvia, where the primitive orders Shell pores have attracted attention from recent Nuculoida and Solemyoida lack a prodissoconch authors and their value for defining certain II, which is present in all higher bivalves (e.g. groups is frequently assumed. Such pores are Jablonski & Lutz, 1980; Gustafson & Reid, known in the shells of Fissurelloidea (e.g. 1986). Bandel, 1982; McLean, 1984; Herbert & A second phenomenon, which is very useful Kilburn, 1986), but are likewise found in Doco- in phylogenetic reconstruction, is the occurrence glossa and other gastropod groups (Tenison- of a hyperstrophic protoconch. This is a wellWoods, 1889; Salvini-Plawen, 1985). Such pores known character of primitive opisthobranchs are also present in several groups of the Bivalvia and pulmonates, but also occurs in certain (e.g. Hudson, 1969; Waller, 1980) and in the groups of the Streptoneura (Allogastropoda, Neopilinidae (Schmidt, 1959; Waren, 1988). see § 4.7.), the affinities of which to the EuthySalvini-Plaw6n (1985) favors a homology neura have often been suggested (Thorson, between the giant epidermal cells of Caudo- 1946; Robertson, 1974, 1985; Haszprunar, foveata, the epidermal papillae of the Soleno- 1985c, d, f). Because such hyperstrophic progastres, the aesthetes of the Polyplacophora toconchs share identical structure (a distinctive and the tube-papillae of the conchiferous helicoid pattern, Be et al., 1972; Richter, 1976), groups. According to this view shell pores are a this character is detectable also in those cases plesiomorphic character. On the other hand, where the hyperstrophy itself is not clearly vissimilar shell pores are present in many brachio- ible due to a more or less direct mode of pods (e.g. Thayer, 1986), thus convergence of development. shell pores among the conchiferous groups or The correlation between a hyperstrophic proeven among the gastropod subgroups cannot be toconch and an orthostrophic teleoconch is excluded. Future fine-structural studies of the described as heterostrophy (angle about 90°) processes themselves are in my view necessary or anastrophy (angle about 180°). Rodriguezto resolve this problem. Babio & Thiriot-Quievreux (1975) suggest a correlation of these types with larval ecology (short or long planktonic larval life), whereas 2.1.6. Larval shells Robertson (1985) disagreed and favoured the Scanning electron microscopy has revealed a influence of the shape of the teleoconch (planigreat deal of new data on the morphology of spiral or high-conical). However, for both views embryonic and/or larval gastropod shells. Aside exceptions can be presented, so the basis of from the considerable importance in species these types remains obscure, as does the phylodetermination, certain conditions of the larval genetic advantage of hyperstrophy itself shell also appear very useful in higher sys- (Robertson, 1985; Haszprunar, 1985c). tematics (Robertson, 1976). Recently, Waren (1988) reported distinct Nearly all marine archaeogastropods are cha- similarities in the mode of loss of the protoconch racterized by the lack of a multispiral larval between Patellidae (Doccoglossa) and the Lepeshell, thus reflecting a nonplanktotrophic tellidae, the most primitive family of the development. Exceptions are the Neritimorpha Lepetelloidea. In both cases the protoconch and which uniquely have three stages of larval shells early teleoconch are actively lost by circular (Bandel, 1982). Also some Pelycidiidae possess dissolution of the shell (see also Bandel, 1982: multispiral protoconchs (Ponder & Hall, 1983), Taf. 8, Figs. 1, 2), analogous to certain probut the anatomy of this family is completely sobranchs such as Caecum or Truncatella, and unknown. Although they possess a rhipido- pulmonates such as Rumina decollate, in which glossate-like radula, their archaeogastropod Hochpochler & Kothbauer (1975) described the mechanism of decollation. nature is not confirmed. In contrast, higher gastropods (Apogastro- 2.1.7. Size poda, Euthyneura) are primarily planktotrophic forms, although there are many exceptions There is agreement among modern paleonshowing yolk-rich and/or direct development. tologists that most early Mollusca (and thus Planktotrophic forms always have a secondary early archaeogastropods) were very small orgalarval shell, thus allowing the classification of nisms of about 1-3 mm (Runnegar & Pojeta,

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1985; Chaffee & Undberg, 1986; Yu, 1987). This is in clear contrast to early Apogastropoda (Loxonematoidea, Campaniloidea, Nerineoidea) all of which are 10-50 cm and thus about 100 times larger. This significant difference is possibly correlated with the mode of larval development (see § 2.6.5.). 2.2. Shell Muscles It is generally accepted that primitive gastropods retain the left and right shell muscle (but see below), whereas most higher gastropods only have a single one. However, there' has been some controversy over whether the single remaining (columellar) muscle is the left or right one*). According to the results of Crofts (1937, 1955) the Trochoidea should have retained the right muscle, whereas higher Gastropoda have retained the left one. This view was criticised by Bandel (1982), who held the opposite view with respect to the Trochoidea. Moreover, there are several cases of higher gastropods with two shell muscles (e.g. Lamellariidae, Rissoellidae) or a horse-shoe-like shell muscle (e.g. Capulidae, Concholepas), and in both cases the homologjes are ambiguous. Using the innervation of the shell muscles these questions were clarified by Haszprunar (1985g). Where a single columellar muscle is present it is always innervated from the left pleural/pedal ganglion and is thus homologous to the left muscle (= velum retractor of the larva), including the Trochoidea. Secondly, the cases of two shell muscles, or horseshoe-like shell muscles, in higher Gastropoda (Apogastropoda, Euthyneura) prove to be the result of secondary divisions or modifications of the single (left) shell muscle. The retention of both shell muscles in the adult (even in certain opisthobranchs two larval retractors occur, e.g. Saunders & Poole, 1910) is a primitive gastropod character found in the Docoglossa and in the rhipidoglossate groups only, although there are certain exceptions such as Neomphalus or most Trochoidea. Since the results of Crofts (1937, 1955) have been used as a basis for phylogenetic considerations (e.g. Fretter, 1969, 1972; SalviniPlawen, 1980, 1981a), the correction of these results likewise influence theories on gastropod evolution. Thus, the suggested difference between the Trochoidea (respectively Vetigastropoda: with solely or predominant 'right' * I tae here tod ebewhere left' md 'rifht' tn the acne of the »dnb

shell muscle) and the higher gastropoda does not exist. However, certain trochoids such as Tricolia Risso, 1826 still have two shell muscles (Marcus & Marcus, 1960; Haszprunar, 1985g). Simply because the loss of the right shell muscle is a process which happened within the Trochoidea, it is not necessary to conclude that the Caenogastropoda evolved from a trochoid stock (see also 5 4.3.). The loss of the right shell muscle appears to be a phenomenon of convergence, probably a later consequence of the evolution of a helicoid shell. In the majority of symmetrical limpets (Docoglossa, group-C limpets; Cocculinoidea) and in certain Neritimorpha (Fretter, 1965, 1984) the horseshoe-like shell muscle is divided into several bundles (Fig. 2A,B). This condition has been compared with that of the Polyplacophora or Tryblidiida (e.g. Harper & Rollins, 1982). This is correct at least with respect to the functional criteria (see §2.5.2.), but is usually rejected for the muscles themselves (Bandel, 1982). Whereas there are several shell muscles in Polyplacophora or Bivalvia (e.g. SalviniPlaw6n, 1980, 1981a, 1984), there seems with evidence from innervation (Haszprunar, 1985g) and ontogeny (e.g. Smith, 1935; Thorson, 1946; Anderson, 1965) to be only a single pair of muscles in the limpets mentioned. However, there are several reasons supporting the possibility of a direct homology of the shell muscle bundles. (1) The larval conditions and the innervation of the shell muscles in the Neopilinidae are not known. Therefore it cannot be excluded that a single larval muscle pair is present in the Tryblidiida. (2) In the Gastropoda the relation between larval and adult shell muscles is quite variable. The adult right shell muscle sometimes does not occur in the larva (e.g. certain Docoglossa; Bandel, 1982), and a right larval shell muscle can disappear during metamorphosis (certain opisthobranchs, e.g. Saunders & Poole, 1910). (3) In Bivalvia, which have several pairs of shell muscles as adults, the larval shell muscles are entirely differently arranged and cannot be homologized with adult pedal retractors (e.g. Meisenheimer, 1900:75). Also pedal innervation does not differ principally from the gastropods (pers. observ. on Nucula nucleus and Striarca lactea). Therefore it is possible that the divided shell muscles in Docoglossa, groupC limpets and Cocculinoidea reflect' a primitive condition. It might also be possible that the 'single' pair of shell muscle in higher gastropods is in fact a fusion of several bundles as assumed by Salvini-Plawen (1981a). This idea is sup-


ported by conditions among the Lepetellidae, where species with divided and solid shell muscles are known (pers. obs.). Also the Neritidae with two or several muscle bundles at each side might represent an intermediate stage. In any case these examples weaken the importance of shell muscle patterns for higher systematics. 2.3. Mantle and subpallial cavity The gastropod mantle cavity is unique among the Mollusca in being anterodorsally situated. It is obvious that this condition is one of the main reasons accounting for the evolutionary success of the class. Moreover, the adaptive radiation of the different gastropod groups is very often correlated with a change of modification of the arrangement of the pallial organs. Thus, a comparative analysis of the gastropod mantle cavity is highly significant with respect to phylogenetics. It is generally believed that the condition of the mantle cavity of the zeugobranch groups, and in particular the Pleurotomarioidea (sensu lato), reflects the most primitive state among the Gastropoda. This can be accepted with respect to the paired elaboration of the pallial organs (osphradia, ctenidia, auricles, hypobranchial glands) and (excretory) openings, but not with respect to the mantle cavity or its organs themselves (see below and § 4.3.). In fact, primitive features of the mantle cavity show a mosaic pattern among different (archaeo-) gastropod groups. Thus, a shallow mantle cavity, a high degree of respiration of the mantle roof and mantle margin and of the subpallial cavity appear as primitive characters. Such conditions are found in the symmetrical limpet groups (Docoglossa, Hot-Vent-C-group, Cocculiniformia). All these groups have a very shallow mantle cavity which is often called a 'nuchal' cavity (Fig. 2A-G). Hickman (1983) also regarded the nuchal cavity as a primitive character but without stated reasons. Indeed, as demonstrated by the example of the Lepetidae, which lack gills altogether, there was no need in a small limpet for a deep mantle cavity to protect large gills; the latter probably were used primarily to produce a water current (see § 2.4.). In contrast, a shallow mantle cavity is much more practicable for water currents than a deep one. Interestingly small secondary archaeogastropod limpets such as sdssurellids or fissurelloids always have rather deep mantle cavities and possess ctenidia. The high degree of subpallial respiration is

375

correlated with direct blood supply by afferent sinuses running between the shell muscle bundles. This resembles conditions known from the Polyplacophora and the Tryblidiida, where the mantle cavity surrounds the shell muscle bundles and includes the ctenidia. Assuming that torsion occurred in animals which were still limpet-shaped as adults (see above), it is likely that the circulatory system could not be transformed at once in such a large step. Thus, the subpallial cavity was used as a respiratory area, and its respiratory capacity could be enlarged by evolving secondary gills (see also below). The Cocculiniformia show how this primary condition could have evolved further. In the Cocculinoidea and certain Lepetellidae the shell muscles are divided into bundles as in the Docoglossa. Other lepetellid species and the more advanced families of lepetelloid limpets, however, have solid shell muscles, and their secondary subpallial gills are supplied by an afferent sinus which surrounds the anterior edge of the shell muscle (Haszprunar, 1987a, 1988a, b). Whereas primitive forms have subpallial gills only, advanced forms have (additional) pallial gill-leaflets. When coiled forms evolved, it was a great advantage to possess a deep mantle cavity, in which the head can be retracted, and this situation is found in the Neritimorpha and all other coiled groups. The prosobranch condition, i.e. the position of the gill in front of the heart is generally accepted as a plesiomorphic character. Thus, phylogenetic relationship cannot be based on this character which is also represented by many primitive opisthobranchs and by the majority of pulmonates (with a lung instead of a gill). Even zeugobranch gastropods possess an asymmetrical arrangement of pallial organs (pericardium to the left, left ctenidium larger, sut to the right; Fig. 2N, O), and the symmetrical condition of the Fissurellidae is generally regarded as secondary (Yonge, 1947; Haszprunar, 1989). This asymmetry is also found in symmetrical limpets and thus is not correlated with the helicoid shell as usually assumed (e.g. Andrews, 1985), but with torsion itself. This is important with respect to the systematic position of the (symmetrical) Bellerophontida, the symmetry of which must be either primitively untorted or secondarily post-torsional. Many authors have agreed that among the gastropods the loss of right-side mantle organs occurred several times for different reasons (Bourne, 1908; Yonge, 1947; Fretter & Graham, 1962; Fretter, 1965; Salvini-Plawen, 1980,1981a). Some recently investigated groups

G. HASZPRUNAR

376

In most aquatic species the main water current (Cocculiniformia, Neomphalus, Melanodrymia, and other Hot-Vent groups) increase the num- of the mantle cavity is produced by the lateral ber of ways in which certain organs of the right cilia of the ctenidial leaflets (see Fig. 1). In many side of the mantle cavity can be reduced. The small caenogastropods (e.g. Truncatelloidea) presence and absence of pallial organs among the ctenidium is more or less reduced, and in the archaeogastropod groups has recently been these cases the water current is produced by summarized by Graham (1985) and is enlarged the enlarged osphradium (e.g. Goetze, 1938; here by inclusion of the new groups in Table 1 Haszprunar, 1985a; Fig. IK). This is in clear and Figures 2 and 3. Obviously the reduction contrast to the allogastropod groups (see Fig. and final loss of the different organs of the right 1L-P), where two opposed ciliary tracts, one at the mantle roof, the other at the mantle side is independent from each other. Salvini-Plaw6n (1980,1981a) assumed the loss floor, create the water currents as of the right pallial organs in the higher gastro- in primitive opisthobranchs (Fig. 1Q) and pods to be caused by paedomorphosis due to a pulmonates. In contrast to Robertson (1985), long-lasting, planktotrophic larval life. How- no reason can be found to reject a homology of ever, the mosaic pattern in the archaeo- these ciliary tracts, since they are present in all gastropod groups, and in particular the con- Allogastropoda and in all aquatic Euthyncura ditions in the pectinibranch Segucnziidae, all which have retained their mantle cavity of which have a short lecithotrophic mode of (Haszprunar, 1985c). In addition, all these development without planktotrophy (except forms have a pallially situated and supplied kidNeritimorpha) contradicts this hypothesis. On ney which serves as an additional respiratory the other hand, the zeugobranchs show a deve- organ. A special solution for the creation of lopmental stage with the left pallial organs only water currents is exhibited by the Valvatidae (Crofts, 1937, 1955). This is more likely one of which use their secondary gill (see below) and the consequences of torsion, causing a primary a densely ciliated pallial tentacle to create a option to reduce organs of the right side. This powerful water current. option improved water circulation in the mantle In many groups of the Caenogastropoda the cavity (Yonge, 1947), and was reached inde- arrangement of pallial organs (anterior-pospendently in several archaeogastropod groups. terior instead of left-right) shows a somewhat TaMe 1. Number of pallial organs, auricles and bdneys In archaeogastropod group*. Hypobranchlal Gland*

Auricles

KUneys

REFERENCES

absent

toft

toft a right

? left

? left

left a right Ml

Pelseneer, 1B99; Thlem, 1917a, b; Fretter a Graham, 1962. McLean, 1985b. Haszprunar, 1887c ISSSd.

absent

toft

M l a right

Haazprunar, 1887a, 1988a, b, d.

laft & right

Ntomp/ulut

l«tt left

laft or none or secondary none left (secondary gill?) secondary gill* left (right vestigial) left left

left (right vestigial) left left

left (a right opening) left laft

LepetodrilokSea

left bright

left

left

toft a right

Bourne. 1909, 1911; Fretter, 1965. 1384; Starmuhlner, 1969. per*, ob*. McLean, 1381; Frettar 1»!.; 1381; pen. ob*. McLean. 1885b. 1988; Fretter, 1988; per*, ob*. Peltsnear. 1899; Bourne, 1910; Haszprunsr, 1989. Fretter a Graham, 1962; Haszprunar, 1989.

ORGANS GROUPS Docoglossa "Hot-Vent-C" Coccullnoldea LapeteltoWea Nertthnorpha MeteoooVym/a

O*phrad!a

Ctenidla

loft a\ right or none ? toft

toft (at laa»t ganglion) laft

left •bsont

SdssureUoid**

left & right

left & right

left a right

toft aright (smaller) toft a right

Ftssurelloldea

l e f t » right

left bright

toft a right

toft a right

left a right

Haliotoldea

M l ft right

left & right

left a right

toft & nQht

left (vestigial) ftrlght toft a right

Pieurotomartoktoa TrochofeJea

toft ft right left

left& right left

toft a right toft a right

toft a right left a right

toft a right toft a right

S«gu«Tuk>td*a CydopnoroMea

left toft or nons

toft absent

left absent

toft toft

toft a right toft

Ampunarloldea

left

left

toft

toft

toft

Croft*. 1929; Fretter a Graham. 1962. Frettar. 1964. 1966. Pelaaneer, 1899; Fretter a Graham, 1962. Qulrtn, 1984; per*, ob*. Tlefecks, 1940 Andrew*. 1981. Annandato a Sewell, 1921; Starmuhlner, 1952,1969.


'detorted' situation (Gainley & Stasek, 1984). This condition can easily be explained as an adaptation to a more or less carnivorous habit. It results in an anterior position of the osphradium, which in these groups is the main chemoreceptor with respect to feeding. In addition, the mantle margin often forms a siphon for sensing the surrounding water currents (see Cross, 1983 for review). Since this 'detorsional' state obviously occurred independently several times in evolution (e.g. Cypraeoidea, Tonnoidea, Stenoglossa), it is clearly the result of convergence. 2.4. Gills Schweigger (1820) created the taxon Ctenobranchia for the prosobranchs, because they have retained true ctenidia. Unfortunately, there are several groups of 'ctenobranch' (= prosobranch) gastropods which have secondary gills (see below), showing that this plesiomorphic character is not diagnostic for all the groups included. In the following discussion the types of gills found in different gastropod groups will be discussed with respect to their homology with the original molluscan ctenidium and their relative primitiveness. 2.4.1. Gills of Vetigastropoda According to Yonge (1947) the ctenidial type of the vetigastropod (especially of the zeugobranch) groups should be the most primitive among the Gastropoda and even among the Mollusca. Whereas this view is accepted by most modern authors (e.g. Fretter & Graham, 1962; Graham, 1985), Salvini-Plawen (1980, 1981a) claimed a somewhat derived condition of the zeugobranch ctenidia, especially with respect to their skeletal rods and the long membranes supporting their axes. Indeed, the ctenidia of the aculiferan classes Caudofoveata and Polyplacophora, which are clearly more primitive than the Conchifera (Salvini-Plawen, 1980,1985; Wingstrand, 1985), lack skeletal rods and efferent membranes. Although modified, the ctenidia of Recent Tryblidiida likewise show this condition (Lemche & Wingstrand, 1959). Secondly, the skeletal rods of the ctenidia of the Cephalopoda, which are most closely related to the Gastropoda (see § 3.1.5.), support the afferent gill-axis and thus clearly have an independent origin. Moreover, Addisoniidae and ChoristeUidae (Cocculiniformia-Lepetelloidea) have skeletal rods in their

377

secondary gill-leaflets (Haszprunar, 1987a, 1988d). Finally, ctenidia of primitive gastropod groups (Acmaeidae, Neritidae, Melanodrymia; Fig. 1A, D,E) lack skeletal rods, and it is unlikely that they have lost these supporting elements. Thus, skeletal rods are a character of convergence, independently evolved in Bivalvia, Cephalopoda, and (twice) in Gastropoda (Salvini-Plawen, 1981a). However, it is likely that skeletal rods have been adopted only once in the true gastropod ctenidium. Nearly the same argument can be used with respect to the efferent membranes. They are not present in the aculiferan and tryblidiidctenidia, support the afferent ctenidial axes in the Cephalopoda, and do not exist in the acmaeid ctenidium. Thus, the ctenidial type of the zeugobranch groups is primitive with respect to number (two) and in being bipectinate. However, it is derived with respect to the skeletal rods, long efferent membranes, and sense organs (see § 2.9.4. and 2.9.5.). Lepetodriloidea and Trochoidea show gradual conditions from a bipectinate to a monopectinate (= pectinibranch) ctenidium. This is initiated by die evolution of a long afferent membrane (McLean, 1984, 1987), which together with the efferent one forms a blind ctenidial chamber. The ctenidial leaflets of this (left) side are less well supplied with incoming water, resulting in a reduction of these leaflets. Also the skeletal rod of the left axis is smaller in these species (e.g. Haszprunar, 1985: Fig. 10). Finally the leaflets of the left side and the respective skeletal rod of the main axis are lost, and a monopectinate condition is reached. These conditions represent a well documented model for the functional reasons for evolving pectinibranch ctenidia. However, it must be stressed that this process happened within the respective groups, and thus cannot be used to trace any relationship between the Trochoidea (or Lepetodriloidea) and the pectinibranch groups. This view is also supported by certain pectinibranch species among the zeugobranch groups such as Fissurisepta (Cowan, 1969) or certain scissurellids (Pelseneer, 1899; Bourne, 1910; Haszprunar 1989). Neomphaloidea and Lepetodriloidea show a remarkably similar modification of the gill. In both cases only a single (left) gill is (at least anteriorly) bipectinate, has a long efferent membrane, and is stiffened by skeletal rods. Since also the position, blood supply, and innervation is typical for true ctenidia there can be no doubt of the ctenidial nature of this gill-type. Analogous to certain filter-feeding trochids

378

G. HASZPRUNAR

B

•g

ct

hg

Fig. 1. Comparative schematic view of the arrangement of organs and openings in the mantle cavity of streptoneiirous and primitive euthyneurous Gastropoda (frontal view, hermaphrodites or females). A— Acmaea (Nacelloidea). B—Cocculina (Coccuhnoidea). C—Osteopelta (Lepetelloidea). D—Septaria (Neritoidea). E—Melanodrymia (Hot-Vent group-A). F—Lepetodrilus (Lepetodriloidea). G—Diodora (Fissurelloidea). H—Gibbula (Trochoidea). J—Bittium (Cerithkridea). K—Rissoa (Truncatelloidea). L—Omalogyra (?Architectonicoidea). M—Hetiacus (Architectonicoidea). N—Glacidorbu (Gladdorboidea), after Ponder (1986). O—Amathina (Pyramidelloidea), after Ponder (1987). P—PyramideUa (Pyramidelloidea), after Ponder (1987) and pers. obs. Q—Acteon (Opisthobranchia).

379

ORIGIN A N D EVOLUTION OF GASTROPODS

N

rd

Ik

ctr

/[

Abbreviations: ct—ctenidium (bipectinate with sensory pockets—monopectinate with skeletal rods); ctr—ciliary tract; gd—gonoduct with opening; hg—hypobranchial gland; Ik—left kidney; os—osphradium resp. ganglion; r—rectum; rd—receptaculum duct; sg—secondary gjlls (plicate—several leaflets); ugd—urinogenital duct with opening. Not to scale.

G. HASZPRUNAR (Umboniinae; see Fretter, 1975; McLean, structure, whereas Thiem (1917b) thought it to 1986), the ctenidia of both groups are modified be the retained anterior portion of an original in having elongated filaments. Based on the (zeugobranch-like) ctenidium. Yonge (1947), existence of a food-groove similar to that of Fretter & Graham (1962), and Salvini-Plawen calyptraeids, McLean (1981) suggested at least (1981a) regarded it as a true ctenidium, and this facultative filter-feeding for Neomphalus also. view is accepted here for the following reasons; However, this view does not fit well with the position, innervation, and blood supply (neradula (see Hickman, 1984a) and the anatomy cessary but insufficient arguments, see below) of the alimentary tract (see Salvini-Plawen & are as in typical ctenidia. In the great majority of Haszprunar, 1987) which are like those of species, the structure of the gill (with alternating detritovores. Neomphalus may also have sym- leaflets having lateral ciliary bands) is identical biotic sulphide-oxidizing bacteria in its ctenidial to those of other primitive ctenidia. The only leaflets as have the Lepetodriloidea (Burgh & exception so far is Rhodopetala rosea (Dall, Singla, 1984) and the bivalves of the Hot-Vents 1872), where the gill is reduced and is repre(Cavanaugh, 1983; Fiala, 1984; LePennec & sented by a simple fold (Lindberg, 1981). MoreHily, 1984). In the Lepetodriloidea the cteni- over, the acmaeid gill closely resembles those dium is further modified in that the left leaflets of the lower molluscan classes in lacking skeletal are much shorter than the right ones (McLean, rods and efferent membranes, being still 1987; Fig. IF). In relation to higher systematic^ contractile. Therefore it is very likely that the all these modifications should be regarded as acmaeid gill type is primitive for the specializations of the respective groups. Gastropoda. Also among the Docoglossa the presence of this gill is a plesiomorphic character. Moreover, it is likely that the loss of the gill has 2.4.2. Gills of Docoglossa happened at least twice (Patellidae, Lepetidae; There is general agreement among authors see§ 4.1.1.). Thus, in accordance with Lindberg that the subpallial gill-leaflets of the Patellidae (1988) gill-conditions should not be used as a and certain Lotriidae are of independent, secon- base for classification of the Docoglossa. dary origin. Their presence or absence is thus not very useful for higher systematics among the 2.4.3. GUIs of Cocculiniformia Docoglossa (Lindberg, 1988). In contrast, the nature of the so-called 'wart- Like the Docoglossa the Cocculiniformia show organ' (tubercle, lymphatic organ) is debatable. great variability with respect to their respiratory These paired organs are present in the majority organs. This is probably due to the additional of docoglossate species, but are lacking in some (plesiomorphic, see above) high degTee of (e.g. Acmaea virginea (Muller, 1776), Patina subpallial/mantle respiration and primary small peUucida (Linnaeus, 1758)), and in the Lepe- size. tidae (Pelseneer, 1899; Thiem, 1917b; HaszpruThe Cocculinoidea (Cocculinidae, Bathynar, 1985a, pers. obs.). Many authors regarded sciadiidae) share a so-called pseudoplicate gill these organs as the rudiments of reduced ctenidia which resembles those of primitive opistho(e.g. Thiele, 1902; Thiem, 1917b; Stutzel, branchs (Fig. IB, Q). Whereas the structure 1984). Recent fine-structural investigations (a simple, pleated fold) of the gill indicates a (Stutzel, 1984; Haszprunar, 1985a), however, secondary origin, its position, innervation, showed in a highly specialized structure com- blood supply, and the distinct similarity with posed of lacunized tissue with possible endo- the gill of Rhodopetala (see above) favour a crine function. Interspersed nervous fibres and ctenidial basis of the pseudoplicate gill in the very specialized cilia with aberrant pattern of Cocculinoidea. Since ontogenetic studies are microtubules are present. The latter structures still completely lacking, the nature of the pseuare also found in the underlying osphradial gan- doplicate gill remains obscure (Haszprunar, glion which innervates the tubercle. It is unlikely 1987c, 1988d). that a ctenidial rudiment can be specialized in The Lepetelloidea (Lepetellidae, Pseudosuch a way, thus the wart-organs are regarded cocculinidae, Pyropeltidae, (?) Bathyphytoas an apomorphic structure of the Docoglossa, philidae, Osteopeltidae, Cocculinellidae, with a possible neuroendocrine function, being Addisoniidae, Choristellidae; cf. Haszprunar, secondarily lost in the above-mentioned taxa. 1988d) primarily have several pallial/subpallial The nature of the pallial gill of the Acmaeidae leaflets of secondary origin (Fig. 2C-H). (in the traditional sense) has been questioned by Additional pallial leaflets are found in many authors. Thiele (1902) regarded it as a secondary genera and these are situated, innervated, and

380


supplied like true ctenidia. However, since the pallia] leaflets occur late in ontogeny and are often reduced, it is more probable that they are likewise of secondary origin and that their presence is a derived condition among the Lepctelloidea. The leaflets of Addisoniidae and Choristellidae, the most advanced lepetelloid families, have skeletal rods in their efferent axes (Haszprunar, 1987a, 1988d). However, due to the secondary nature of the leaflets, also these skeletal rods are regarded as secondary structures (see above). 2.4.4. Gills of Neritimorpha and Melanodrymia Certain authors have doubted the ctenidial nature of the neritid gill (e.g. Bourne, 1908; Thompson, 1980). This view is mainly based on the erroneous statement that the innervation of the gill is from the left pleural instead of from the supraoesophageal ganglion, an impression gained because of the considerable concentration and modification of the neritid nervous system (Boume, 1908; Starmuhlner, 1969). Because the arrangement and structure of the leaflets are typical for ctenidia (Fig. ID), the neritid gill is accepted here as a true ctenidium. Its lack of skeletal rods and contractibility are primitive conditions (Bourne, 1908; Yonge, 1947; Fretter, 1965; Starmuhlner, 1969; SalviniPlaw6n, 1980). Being equipped with a fairly long efferent membrane, the neritid gill represents an intermediate type between the acmaeid and the zeugobranch type. The same type of gill is present in Melanodrymia awantiaca Hickman, 1984 (Fig. IE), a small coiled gastropod from the hydrothermal vents of the East Pacific (pers. obs.). However, according to Fretter (pers. comm.) other hotvent gastropods with the same radula type (group A of Hickman, 1983) have skeletal support of the ctenidium. The structure and innervation of the gills of Melanodrymia (no data on other group A species) show this is a true ctenidium. 2.4.5. Monopectinate (= pectinibranch) gills Most aquatic species of the Seguenziidae, Ampullarioidea, and Caenogastropoda possess a monopectinate gill. Because of distinct structural similarities (skeletal rods, lateral cilia), and identical position, innervation and blood supply, its ctenidial nature cannot be doubted. In contrast to the primitive groups there is a remark-

381

able uniformity of the ctenidium in the pectinibranch groups. Specific modifications occur with respect to filter feeding (elongation of the leaflets) e.g. in the Calyptraeidae. Certain small Truncatelloidea more or less reduce their ctenidium, and in these cases the enlarged osphradium produces the water current (see e.g. Goetze, 1938; Haszprunar, 1985a; Fig. IK). In terrestrial forms the ctenidium is lost and replaced by a 'lung' analogous to that of the pulmonates.

2.4.6. Ectobranch gills Woodward (1899), Moore (1972), Graham (1982), and Bieler & Mikkelsen (1988) described the interesting gill of Vitrinellidae and Tornidae (often united as a single family of the Truncatelloidea). The gill is situated to the right of the mantle cavity. Its efferent membrane is rather short, the anterior portion hangs free into the mantle cavity and shows a bipectinatc condition. So far as is known (Bieler & Mikkelsen 1988) skeletal rods are lacking. Therefore the ctenidial nature of the vitrinellid/tornid giU is doubtful. Based on the similarities of their gills, the Vitrinellidae/Tornidae and the Valvatidae were united in a single taxon Ectobranchia by Golikov & Starobogatov (1975). The valvatid gill has been regarded by nearly all authors (Bernard, 1890; Yonge, 1947; Starmuhlner, 1952; Fretter & Graham, 1962,1978) as a retained bipectinate ctenidium. Indeed, gill conditions of the common European species Valvata piscinalis MQUer, 1774 and V. cristata MQller, 1774 (bipectinate, devoid of efferent membrane and skeletal rods) seem to favour this view, although Yonge (1947) reported intraspecific variation (a single specimen with doubled left leaflets), a very rare phenomenon among gastropod ctenidia. Salvini-Plawe'n (1981a) also noted the unusual total dliation of the valvatid gills and regarded it as a distinct type of ctenidium independently evolved from a primitive one. Recent investigations of the development of the gill in V. piscinalis and of the gill morphology of other valvatid species (Rath, 1988) revealed that the valvatid gill is a secondary structure because of the following facts: (i) There are large differences between the gills of different species. Many species have a gill consisting of two main axes with interconnecting leaflets, (ii) Several intraspecific aberrant speciments of V. piscinalis and V. cristata have been found, (iii) The valvatid gillfirstappears at hatching and thus much

382

G. HASZPRUNAR

later than typical ctenidia of other freshwater species. Moreover, regarding the valvatid gill as a primitive ctenidium would imply a convergent evolution of the taenioglossate radula and of the epiathroid nervous system in the Valvatidae. This assumption is much less probable than a secondary nature of gill, especially in the light of the facts mentioned above. The case of the valvatid gill demonstrates that position, innervation, and blood supply alone are not sufficient characters to establish a gastropod gill as a true ctenidium. 2.4.7. Foliobranch gills of Allogastropoda Robertson (1974) created the term foliobranch for the gill leaflets of the Architectonicidae, but he did not discuss the homology of this gill type. Based on detailed histological examination of several architectonicids, mathildids, and a large pyramidellid, the author showed that these leaflets are specialized portions of the hypobranchial gland (Haszprunar, 1985b, c, d, f; Fig. 1M, O, P) and thus clearly have a secondary origin. Meanwhile Ponder (1988a) described the anatomy of Amathina, representing a new pyramidelloid family. In this case also the structure and blood supply show the gill leaflets to be secondary. However, the gill leaflets of Amathina are situated (as in euthyneurans) to the left of the dorsal ciliary tract (Fig. 10, Q), whereas they are situated to the right in the Architectonicidae, Mathildidae, and Pyramidellidae (Fig. 1M, P). Also a (still undescribed) pyramidellid from New Zealand deep-waters has the gill-leaflets at the left side (Haszprunar, unpubl.). Contiary to previous assumptions (Haszprunar, 1985c, d, f ) this makes a homology of the gills of the Architectonicidea and Pyramidellidae unlikely. The analogous character of the foliobranch gills is additionally supported by the fact that small species of all allogastropod groups (Omalogyridae & Architectonicoidea, Glacidorbidae, Rissoellidae, Pyramidelloidea; Fig. 1L, N) generally lack gills at all, and that the mathildid Gegania valkyrie Powell, 1971 has additional (secondary) leaflets near the heart (Haszprunar, 1985f). These conditions clearly show that the original ctenidium has been lost in the allogastropod (and subsequent euthyneuran) ancestors. The respiratory function is replaced by the pallially situated and supplied kidney and (in large species) by multiply evolved secondary foliated gill leaflets. The function of the ctenidium with respect to water currents is replaced by a pair of opposed ciliary tracts which are present in

all allogastropod groups (Robertson, 1985; Haszprunar, 1985c, d, f). In the Architectonicidae, Mathildidae, and Omalogyridae (pers. obs. on six species of two genera) these are situated on the left side of the mantle cavity (Fig. 1L, M), thus producing an inhalant current. In contrast, Rissoellidae (Fretter, 1948; pers. obs. on two species), probably the Glacidorbidae (see §4.7.5.), Pyramidelloidea (Fretter & Graham, 1949; Robertson, 1985; Haszprunar, 1985c, Ponder, 1988a), and (primitive) Euthyneura have them on their right side (Fig. 1N-P), where they produce an exhalant current. This may be correlated with the fact that in the latter groups the gonoducts are placed at the floor of the mantle cavity, whereas they are retained at the mantle roof in the groups with left ciliary tracts. In any case the presence of ciliary tracts and their relative position serve as important synapomorphic characters at the allogastropod level of evolution (see § 4.7.).

2.4.8. Plicate gills of Euthyneura (= Pentaganglionata) There is a long-standing debate on the ctenidial nature of the plicate gill which is found in most of the primitive opisthobranchs. Most authors regard the plicate gill as a true ctenidium (Hoffmann, 1940; Schmekel, 1985), whereas others claim an independent, secondary origin (Morton, 1972; Haszprunar, 1985c). The first view is founded on the position and innervation of the plicate gill, but as shown in the case of the valvatid gill, these characters are not sufficient. Blood supply of the plicate gill (performed from the pallial caecum, Brace, 1977b) differs significantly from that of the caenogastropod ctenidium (from the kidney; Fretter & Graham, 1962; Andrews, 1985). Moreover, this gill is used for respiration only, since the water current is produced as in allogastropods by the two opposed ciliary tracts (e.g. Fretter & Graham, 1954). In addition, the structure of the plicate gill (one pleated fold or several simple folds; Fig. IP) is entirely different from that of the pectinibranch ctenidium (it is very improbable that the Euthyneura arise directly from an archaeogastropod stock, see § 4.8.). Finally, all forms, which interconnect Caenogastropoda

and primitive opisthobranchs, possess secondary gills (ecto- or foliobranch) or no gills. Summing up, there are many reasons to regard the plicate gill as a secondary structure. Its ctenidial position and innervation can easily be explained by functional reasons, as in the


383

apomorphic) situation of Allogastropoda and Euthyneura. As outlined in detail by Brace (1977b), conditions in the (primitive) Euthyneura are even more modified. Here the shell muscle forms a 2.5. Heart, circulatory and excretory system transverse septum, and an additional respiratory area in the anterior mantle roof is developed, 2.5.1. Heart which is homologous to the lung of the pulConditions of the heart itself are of minor phylo- monates (compare Brace, 1977b and Brace, genetic significance. It is generally accepted that 1983). a diotocard heart with two functional auricles represents the most primitive condition. Among 2.5.3. Excretory system Recent Gastropoda these features are only found in the vetigastropod groups, and even The archaeogastropod groups show a remarkhere in the Trochoidea and Lepetodriloidea, the able diversity with respect to their excretory right ctenidium has been lost. In addition, a system, and its special elaboration is again typivestigial, right auricle is retained in certain Neri- cal for each group (Fig. 2). It is obvious that the itidae (Bourne, 1908; Fretter, 1965), but has presence of two kidneys* is a primitive character among the Gastropoda. However, there are apparently lost its function. several archaeogastropod groups with a single (left) kidney only (Cocculinoidea, Neritimorphia; Melanodrymia, Architaenioglossa), and 2.5.2. Circulatory system the distribution of this condition shows that the There is considerable variation with respect to loss of the right kidney is a character of the circulatory system among primitive gas- convergence. Thus, presence or absence of the tropod groups and nearly every archaeogas- right kidney can well be used to define an archaetropod superfamily has its own type (Fretter & ogastropod group, but is no argument with Graham, 1962; Andrews, 1981, 1985). Here respect to its interrelationships. only such characters are considered which are Where the right kidney has been retained, it regarded as important for determinating is always different in size, shape, and structure relationships between the main groups. As men- from the left one (Andrews, 1981, 1985; Hasztioned above the symmetrical limpet groups pos- prunar, 1987d, 1988b, c, unpubl.). Andrews sess shell muscles which are divided into several (1985) reported several fine-structural differbundles. Blood coming from the haemocoel (or ences between the kidneys of Patellidae and the right kidney) passes between these bundles those of the vetigastropod groups. Kidney fineand reaches the subpallial cavity and the mantle structure might be useful for phylogenetics, but margin which may or may not be equipped with additional investigations are necessary on the special respiratory structures. Based on the other groups to substantiate its significance. apparent similarity of this mode of circulatory If two kidneys are present, the left one is system with those of Polyplacophora and Recent usually smaller and is situated in the mantle Tryblidiida, itis likely that it represents a primi- roof, whereas the right one mainly occupies tive condition rather than a secondary modi- the visceral hump (Docoglossa, Lepetelloidea, fication as is usually assumed (for the homology Fissurelloidea, Lepetodriloidea; Fig. 2A, C-H, of muscle bundles, see § 2.2.). M-O; probably also Hot-vent group C, see Starting with primary coiled forms (Neriti- below). In contrast to earlier reports, (Quinn, morpha), it appears to be a general rule that 1984; Salvini-Plawdn & Haszprunar, 1987) also the blood from the haemocoel of the animal's the Seguenziidae possess two kidneys (Fig. body is at first filtered by the (right or single 2P). Recently, the Pyropeltidae (Cocculinileft) kidney, and then is oxygenated by passing formia-Lepetelloidea), a family from the hydrothe respiratory organs. This is in clear contrast thermal vents, has been described. In this family to the conditions of the allogastropod and euthy- the left kidney is vestigial (McLean & Haszpruneuran groups, where a (pallially situated, see below) kidney is supplied by blood coming from I use tbe tens 'kidney* because of familiarity. However, it ibouM the mantle cavity and acts as an (additional or be *stressed that this type of excretory organ h tbe derivative of the solely) respiratory organ (Brace, 1977a, b, 1983; orifinal pericardia! duct occurring first at the level of Poryplacopbon (13 I.)It is thus DOC homolofons with the 'kidneys' or 'nephridia' of Fretter, 1978; Haszprunar, 1985c, d, f). The lat- any other phylum, but appear to be an evolutionary novelty within tbe ter is obviously a commonly derived (syn- moOuscan line (e.g. Sahvml-Plawen, 1961a, 1985). case of the (pallial) cocculiniform or valvatid gills.

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G. HASZPRUNAR

Fig. 2. Comparative semiscbematk view of the gill-types and gonopericardial (receptacula are omitted) system of archaeogastropod (less architaenioglossate) groups. A—Acmaea (Nacelloidea), female. B— Cocculxna (Cocculinoidea). C—Lepetella (Lepetelloidea). D—Pseudococculina (Lepetelloidea). B— Pyropelta (Lepetelloidea). F—Osteopelta (Lepetelloidea). G—Addisonia (Lepetelloidea). H—Chorisulla (Lepetelloidea), female. J—Nerita (Neritoidea), female, the large pericardia] ducts are omitted, the shell muscle consists of distinct bundles. K—Melanodrymia (Hot-Vent group A), female. L— Neomphatus (Neomphaloidea), female. M—Lepctodrilus (Lepetodriloidea), female. N—n.fam, n.gen., n.sp. (Fissurelloidea), female. O—Indsura (ScissureUoidea), the right ctenidium is monopectinate, female (after Bourne, 1910). P—Tricolia (Trochoidea), female. Q—Carenzia (Seguenzioidea), female.


Abbreviations: a) pseudoplicate gill; b) secondary leaflets with sensory pockets, lacking skeletal rods; c) secondary leaflets with skeletal rods and specific glandular areas; d) bipectinate ctenidium with sensory pockets (bursicles), lacking skeletal rods; e) monopectinate ctenidium with skeletal rods; gd—gonoduct (respectively vas deferens or oviduct); Ik—left kidney; o—ovary; pc—pericard with auricle and ventricle; r—rectum; rk—right kidney; sm—left and right shell musde; te—testis. Not to scale.

385

386

G. HASZPRUNAR

nar, 1988; Fig. 2E). In most vetigastropod groups the left kidney is specialized to a socalled papillary sac. Most authors regard this condition as the most primitive amongst (archaeo-) gastropods (Fretter& Graham, 1962; Andrews, 1981, 1985; Graham, 1985). However, in the light of the primitiveness of the Docoglossa (see §4.1.1.1.) and the identical conditions in the likewise symmetrical lepetelloid limpets (Haszprunar, 1988d), it is much more likely that the conditions of these groups are the most primitive in the Gastropoda. Thus, the papillary sac of Vetigastropoda (the somewhat reduced left kidney of the Fissurelloidea has essentially the same fine structure; Andrews, 1985) is now regarded to be an apomorphic character of this group (see also § 4.3.). These arguments should be strengthened by the conditions to be found in the new archaeogastropod groups from the hypothermal vents. In his preliminary report McLean (1985b, based on preliminary results of Vera Fretter) stated for the group-A 'there are left and right kidneys'. Meanwhile, however, Fretter (pers. comm.) states that there is only a single (left) kidney as in Melanodrymia aurantiaca Hickman, 1984 (Fig. 2K). In the Lepetodriloidea a papillary sac was originally reported (McLean, 1985b), but Fretter (1988) stated that the left kidney is not like a papillary sac, though different from the right one (Fig. 2M). In the group-C 'two kidneys are present; the gonad discharges via the duct of the right kidney' (McLean, 1985b); no further commentary is given so far. Kidney conditions of Neomphalus seem debatable. After earlier statements on a vestigial left kidney, McLean (1981: p. 325, footnote 11) and Fretter et al. (1981) concluded that the single kidney of this aberrant species is the left one. I have checked serial sections of Neomphalus fretterae and fully agree with this view (Fig. 2L). As in Melanodrymia the kidney of Neomphalus forms a large cavity and includes concentrically structured concTement bodies. As already outlined, the Allogastropoda and the Euthyneura have their (single left) kidney pallially situated (Fig. 2L-Q). The same condition is found in several freshwater or terrestrial streptoneuran groups (Andrews, 1981), but in all these cases the blood supply is visceral (instead of pallial in the Allogastropoda and Euthyneura, see above). 2.6. Genital system and reproduction There can be no doubt that the genital system as a whole as well as gamete conditions are

extremely useful in tracing phylogenetic relationships at nearly all levels of gastropod systematics. Here consideration is restricted to those characters which appear important for higher streptoneuran systematics. 2.6.1. Anatomy of genital system In all Recent Gastropoda only a single (right) gonad is present. This is often thought to be one of the consequences of torsion. Asymmetry of the genital system, however, is likewise found in the Scaphopoda and Cephalopoda (Decabrachia), and it might be the result of space problems in their elongated bodies. Therefore the correlation with torsion is uncertain. It is generally accepted that the release of the gametes 'through the right kidney' reflects the most primitive condition among the Gastropoda. However, there are distinct modifications in different archaeogastropod groups (Fig. 2). So far it is known (hot-vent group-C?) that the gonad opens directly into the right kidney only in the Docoglossa (Fig. 2A). In the Vetigastropoda the gonoduct opens into the renopericardial duct (Lepetodriloidea, Fissurelloidea; Fig. 2M, N) or adjacent to this duct (all other groups with retained right kidney: Fig. 20, P) (Pelseneer, 1899; Fretter & Graham, 1962; Andrews, 1981, 1985). In the latter cases the renopericardial duct meets the right kidney at its common opening. Also the genital and excretory systems of primitive Lepetelloidea (Lepetellidae, Pyropeltidae, Pseudococculinidae) unite distally near the common opening. Lepetellidae (Fig. 2C) have retained a common releasing chamber, whereas in Pyropeltidae and Pseudococculinidae genital and excretory ducts unite simply (McLean & Haszprunar, 1987; Haszprunar, 1988a, b, d; Fig. 2D,E). Similar conditions are known in the Bivalvia, where a continuum of intermediate types between these modifications exists (Pelseneer, 1899; Mackie, 1984). Such modifications appear easily possible, since the cavities of the pericardium, excretory and genital system are derivatives of the same (4d) cell. The coelomic* conditions in the Recent Tryblidiida are not yet known in detail (Wingstrand, 1985). Also ontogenetic data on the different modifications are needed to ascertain which of these modifications is the most primitive one. From the functional point of view (increasing degree of specialization) the * The mnflhnran coelom b doubtful in hi homologjr to that of other protostoaie phyla. Accenting to Sahrini-Plawen (1985) and Winptrand (1983) it is regarded as a tnie coelom with raped to origin (ceO 4d) and Uiumuc (maepitbelUl cavity), but in bomologjr with the coelomi (as organs) of other phyla a rejected.


docoglossate type, where the gonoduct opens directly into the right kidney, is the most primitive for the Gastropoda. This type is also present in primitive Bivalvia (Protobranchia), and thus might be ancestral for the Conchifera. However, it is certainly not ancestral for the Mollusca, because the most primitive groups Caudo foveata and Solenogastres lack specific excretory organs. It has become nearly a dogma that a distal visceral portion of the gastropod gonoduct has always been developed by incorporation of the right kidney. Therefore this portion is usually called the renal gonoduct (e.g. Fretter & Graham, 1962; Fretter, 1984b). In fact, certain other features seem to support this theory. In primitive marine neritids a (functionless?) kidney opening is still present (Fretter, 1965,1984), and in many caenogastropods there is a connection between the genital system and the pericardium. However, bearing in mind the identical origin of the pericardium and gonocoel, such conditions are not convincing. The theory becomes more weakened by the recently described features of the advanced lepetelloid famines (Osteopeltidae, CocculineUidae, Addisoniidae, Choristellidae; see Fig. 2F-H) and of the Seguenziidae (Fig. 2P), where a right kidney and (a) true gonoduct(s) with separated openings exist. With the sole exception of the Docoglossa, a renal gonoduct is not present in those archaeogastTopod groups where the right kidney is retained. This demonstrates that a renal portion of the gonoduct is not necessarily present in an archaeogastropod. The same trend is also detectable in the Bivalvia (Pelseneer, 1899; Mackie, 1984). In all these cases such non-renal gonoducts are characterized by a total lack of accessory glands or vesicles; they form simple, ciliated tubes. The loss of the right kidney as an excretory organ and its incorporation into the gonoduct is without doubt a matter of multiple convergence occurring in the Cocculinoidea, Neritimorpha, Melanodrymia, Neomphalus (Fig. 2B, J-L), and a higher Gastropoda. Despite their independent origin, renal portions of gonoducts are generally glandular. A pallial gonoduct, which by definition originated from the pallial wall (Fretter & Graham, 1962; Fretter, 1984b), is seldom found among the archaeogastropods. Such pallial portions, forming longitudinal folds, pouches OT distinct areas, are present in the hermaphrodites Problacmaea (Golikov & Kussakin, 1972) and Cocculinella (Haszpninar, 1988a), in males of a new fissurelloid family (Haszprunar, 1989), in

387

females of Pleurotomariidae and Trochoidea (Fretter, 1964, 1966), in both sexes of the Neritimorpha (Fretter, 1965, 1984a), Lepetodriloidea (Fretter, 1988); Melanodrymia (pers. obs.), and Carenzia (pers. obs.). All these paltial portions are probably of independent origin. It may be assumed that such pallial gonoducts are the derivatives of the right hypobranchial gland. This might be correct in the case of higher gastropods, but is certainly wrong in the cases of Neritimorpha and the vetigastropods mentioned, where a right hypobranchial gland is still present (Fig. ID). This assumption is also improbable for Problacmaea and Cocculinella, because their respective superfamilies lack hypobranchial glands (see Table 1). Once more, ontogenetic studies are badly needed to ascertain the origin of the different portions of the gonoducts in all lines of gastropods. Most modern authors think that gonochorism is the primary condition among the Gastropoda. However, there are several Docoglassa showing protandric hennaphroditism, and in the Cocculiniformia a step-like evolution from hermaphroditic to gonochoristic forms can be reconstructed (Haszprunar, 1988d). Recently, Nakamura (1987) claimed a hermaphroditic gastropod archetype on the base of comparative chromosome morphology. Also many Caenogastropoda are protandrous hermaphrodites. Again these conditions parallel those of the Bivalvia, where different trends (from gonochorism to hennaphroditism or vice versa) can be found in different lines. Mackie (1984) concluded that a protandric hermaphrodite probably was the most primitive condition in the Bivalvia, which gave rise to gonochoristic forms or to simultaneous hermaphrodites, and the same might be true for the Gastropoda. In any case, numerous occurrences of protandric or simultaneous hermaphroditism among the streptoneurous Gastropoda demonstate that sexual conditions have no significance at all in tracing phylogenetic relationships at higher levels. There can be no doubt that free fertilization primitively occurred in the Gastropoda. Obviously internal fertilization has been developed by multiple convergence. Even Docoglossa or Vetigastropoda, the members of which generally have external fertilization, include certain exceptions such as Problacmaea (Golikov & Kussakin, 1972; Lindberg, 1979), the new fissurelloid family from the hot-vents (Haszprunar, 1989), or the Skeneidae (Fretter & Graham, 1976; pers. obs.). There is a distinct trend in archaeogastropod groups to use the

388.

G. HASZPRUNAR

(left or right or both) cephalic tentacle or a nearby situated process as copulatory organ, which is thus cerebrally innervated. In contrast, copulatory organs of higher streptoneurans ( = Apogastropoda) are in most cases of pleural or pedal origin (Fretter & Graham, 1962). Contrary to earlier statements (e.g. Fretter & Graham, 1962; Fretter, 1984b), Fretter (1984a, pers. comm.) now agrees that the primitive situation in the apogastropod grade is probably represented by an aphallic condition which is still retained in the Cerithioidea (for the primitiveness of Cerithioidea see §4.6.1.), Triphoroidea, Janthinoidea, Vermetoidea,- and Architectonicoidea. In all these groups sperm transfer is achieved by using (motile) spermatophores alone. Consequently the copulatory organs of the different caenogastropod, ectobranch and allogastropod groups cannot be homologous with each other. Only within the superfamilies themselves (although according to Ponder (1988) a penis arose twice in Truncatelloidea) and among the Euthyneura can the penis be homologized throughout like the genital system as a whole (Ghiselin, 1966; Gosliner, 1981). In nearly all streptoneuran Gastropoda, which have a penis, this is an external and contractible organ. By contrast, Glacidorbidae, certain Pyramidelloidea and Euthyneura (few exceptions such as the Acteonoidea) can retract the whole copulatory organ. The latter condition is without doubt the more derived one. However, the variability of the genital apparatus among the Pyramidelloidea (Fretter & Graham, 1949; Robertson, 1978; Ponder, 1987; see also § 4.7.6.) suggests parallelism of this phenomenon at least within this superfamily. Much attention has been paid to the occurrence of open pallial gonoducts among the Caenogastropoda by earlier authors (e.g. Fretter & Graham, 1962). Contrary to earlier opinions (Fretter, 1984b) Fretter (1984a, pers. comm.) now agrees that the primitive nature of this condition among the apogastropods can no longer be doubted both because of distribution (nearly all cerithioids) and ontogenetic evidence (dosing of gonoduct during ontogeny recapitualates phylogeny). Nevertheless it is of little use for phylogenetic reconstruction. Closing of such an open groove to form a duct is always very easily possible during ontogenesis, therefore a multiple process can be assumed. This is additionally supported in the many cases where the gonoduct is closed in one sex only (see Webber, 1977; or Fretter, 1984b for a review). In the great majority of streptoneurous

gastropods the (open or closed) pallia! portion of the gonoduct is situated in the pallial roof (Fig. 1J-M). In contrast, Rissoellidae, Glacidorbidae, and Pyramidelloidea have their pallial gonoducts lying in the pallial floor, a situation also found in the most primitive Euthyneura (Acteonoidea, Ringiculoidea, Diaphanoidea; see Haszprunar, 1985c; Fig. 1N-Q). 2.6.2. Spermatozoa It has been well known for a long time that there are considerable differences in the shape, structure, and development of gastropod sperm. It has been one of the goals of fine structural research of the few last decades to ascertain these differences, thus revealing one of the main new character sets to be included in phylogenetic reconstruction of the Gastropoda. A series of comparative papers on this subject have been published by different authors (Nishiwaki, 1964; Giusti, 1971; Thompson, 1973; Kohnert & Storch, 1984a, b; Koike, 1985; Healy, 1988). The spermatozoa of Docoglossa and Vetigastropoda resemble each other superficially, since (most) members of both groups still have external fertilization (sperm morphology of species with internal fertilization is yet unknown). However, there are considerable differences in sperm fine-structure (Lewis et al., 1980; Azevedo, 1981; Kohnert & Storch, 1983; Hodgson & Bernard, 1988) which reflect the large phylogenetic distance between both groups. Since the sperm ultrastructure of the Recent Tryblidiida as well as of several newly or re-discovered archaeogastropod groups is still unknown, no attempt should be made at present to decide upon the most primitive condition in detail. Sperm-dimorphism has been reported in Patella caendea Linnaeus, 1758, but only 1% of its spermatozoa are of atypical shape (Indelicato & Streiff, 1969). This dimorphism is a wellknown phenomenon in most (but not all) streptoneuran groups with internal fertilization (for an overview see Webber, 1977; Melone et al., 1980; Giusti & Selmi, 1982). High specialization of these paraspermatozoan characters support their use in tracing phylogenetic relationships. In his recent review Healy (1988) emphasized the great differences of the paraspermatozoa of the Neritimorpha and those of all other Gastropoda suggesting 'that both groups originate from different archaeogastropod ancestors'. Architaenioglossa and Cerithioidea (and Campanile symbolicum Iredale, 1917) share the same type of paraspermatozoa, reflecting a distinct grade


and the primiti veness of the Cerithioidea among the Caenogastropoda (Healy, 1983a, 1986a, b). Also among the Caenogastropoda fine-structure of paraspermatozoa reflects distinct relationships, although further investigations are necessary. For instance, so-called spermatozeugmata have been reported in Triphoridae and Cerithiopsidae as well as in Epitoniidae and Janthinidae (Healy, 1988), reflecting a dose relationship between these families. Stenoglossa (= Neogastropoda) share the same type of paraspermatozoa as Calyptraeoidea, Cypraeoidea, Tonnoidea, and Naticoidea (Nishiwaki, 1964; Healy, 1986c, 1988). Valvatoidea, Allogastropoda and the Euthyneura lack paraspermatozoa (there are certain exceptions; Schmekel 1985). This might be because of the primary simultaneous hermaphroditism occurring in these groups. Also the fine-structure of typical or eupyrene or euspermatozoa has revealed several apomorphic characters for various groupings as recently summarized by Healy (1988). Interesting enough, sperm-morphology generally connects the gaps resulting from major changes of the nervous system (see § 2.9 and 5.2.). Thus, Cyclophoroidea, Ampullarioidea, and Cerithioidea show several distinct similarities in their euspermatozoan morphology, suggesting (as the paraspermatozoa, see above) the basal position of the latter among the Caenogastropoda. All other Caenogastropoda (including the Vermetoidea as a superfamily proper) have the same type of euspermatozoa (Healy, 1988). Moreover, sperm-conditions have become one of the main characters in tracing the transitory field between Caenogastropoda and Euthyneura. As recently outlined in detail by Healy (1986b) the euspermatozoa of Campanile symbolicum Iredale, 1917 also show the isolated position of this relict species among the primitive pectkubranchs (paraspermatozoa are like those of other certithioids). As earlier noted by Healy (1982) the spermiogenesis of the Epitoniidae is entirely different from that of the Architectonicidae, thus supporting the total separation of these families by Haszprunar (1985ad, f) in contrast to Robertson (1974, 1985). Allogastropoda and Euthyneura, however, share the mode of acrosome and midpiece development and the type of nuclear condensation. Moreover, Rissoellidae, Pyramidelh'dae and Euthyneura have in common paracrystalline material and glycogen helices in their midpieces (Thompson, 1973; Healy, 1982, 1988), suggesting an even closer relationship of the latter families with the Euthyneura.

389

2.6.3. Eggs and spawn Size, number and structure of gastropod eggs are closely correlated with reproductive biology and thus of little significance for higher systematics. However, one egg-character is highly valuable for phylogenetics, the so-called chalazae. These egg-strings interconnect single eggs, therefore similar structures of the Epitoniidae, which connect egg-capsules with several eggs within, cannot be homologized with the chalazae. Chalazae are found in the cerithioid-like relect species Campanile symbolicum Iredale, 1917, in the Valvatidae, in the Architectonicoidea and Pyramidellidae, and are generally known in primitive euthyneurans. Recently Robertson (1985) presented a detailed review of their occurrence, but claimed doubts on their homology with the egg-strings of Valvatidae and of the higher limnic Basommatophora because of structural differences. However, since there are certain differences also between doubtless homologous chalazae (Robertson, 1985) the reported small differences in the latter groups appear of little importance and might be due to their freshwater habitat. Moreover, regarding the valvatid eggstrings as true chalazae perfectly agrees with the proposed systematic position of this family according to other organ systems (see §4.6.). Detailed structural investigations on the chalazae appear necessary, however, to ascertain their homology throughout the Gastropoda. Also the homology of the egg-strings of Campanile symbolicum has been doubted by Robertson (1985). However, the recent cladistic analysis of the Cerithioidea (Houbrick, 1988) and data on sperm-morphology (Healy, 1986b) and osphradia (§ 2.9.4.) likewise revealed that Campanile is the first offshoot of the group. Again the assumption of homologous chalazae corresponds well with the proposed systematic position (see §4.5.1.).

2.6.4. Larvae The value of the gastropod protoconch for phylogenetics has already been outlined (see § 2.1.5.). As a special larval organ the so-called echinospira-larva occurs in certain caenogastropod families (Fretter & Graham, 1962, 1980). This secondary larval 'shell' has been reported in Capulidae (incl. Trichotropidae, see Ponder & Wartn, 1988), Lamellariidae, and Triviidae (summarized by Webber, 1977),

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reflecting distinct affinities between the respective families. There has been a recent debate by Robertson (1985) and Haszprunar (1985c) on the homology of the so-called pigmented mantle organs (PMO = 'black larval kidneys'). Such organs have been reported from the Janthinoidea, from the AUogastropoda and Opisthobranchia. Whereas there is agreement on the homology of PMOs of AUogastropoda and Opisthobranchia, Robertson (1985) also proposes to homologize the PMOs of the Janthinoidea with those of the other groups. However, the janthinoid PMO is in fact a larval hypobranchial ( = purple) gland (Richter & Thorson, 1975), whereas it is probably an excretory organ of separate origin in the remaining groups (e.g. Bonar & Hadficld, 1974; Bickell & Chia, 1979). Therefore both types of PMO cannot be homologous. The analogy of the PMOs is supported by the fact that Janthinoidea and AUogastropoda are only very distantly related (see Haszprunar, 1985a-d, f; § 4.7.). It is interesting to note that the allogastropods generally retain their 'larval kidneys' in the adult (also Omalogyridae and Rissoellidae pers. obs.), whereas it is only a larval organ in the euthyneuran groups. This might be due to paedomorphosis or reflects the status of the allogastropod grade as an ancestral one with respect to the Euthyneura. Robertson (1985) doubted the existence of PMOs ('larval kidneys') in the Pulmonata. However, they are reported by Fretter (1943) for Onchidiidae, by Little et al. (1985) for Amphibolidae, by Mapstone (1978) for Siphonariidae, by Haven (1973) for Trimusculidae, and by Berry (1977) and Ruthensteiner (in Myosotella myosotis (Draparnaud, 1805), pers. comm.) for EUobiidae. Thus, PMOs are quite common among primitive pulmonates, supporting as do many other characters (see § 4.8.) the evolutionary unity of the Euthyneura (Haszprunar, 1985c). 2.6.5. Development The results on the larval shells and the eggconditions of archaeogastropods clearly show that the primary mode of gastropod development is lecithotropic and not planktotrophic. Olive (1985) in a general review on invertebrates, and Chaffee & Iindberg (1986) in a study on early Conchifera independently resulted in a correlation of non-planktotrophy and small size in the case of external fertilization. The latter authors even claimed a non-planktic mode of development. This is

unlikely, however, because all primitive conchiferan groups possess larvae (pericalymma type or pseudotrochopora type; Salvini-Plawdn, 1981a, 1985). The biotic conditions of the Cocculiniformis and the hot-vent groups (habitat islands) also demonstrate, however, that species with non-planktotrophic development can overcome large distances (Turner et al., 1985; Haszprunar, 1988b, d). The advantage of evolving planktotrophic larva was not solely greater dispersal capacity, but was also correlated with the existence of large animals (see §2.1.7). This may have enabled the early Neritimorpha and especially the Apogastropoda to invade new habits and habitats and enhanced the evolutionary success of the group.

2.7. Alimentary tract 2.7.1. Radula Scanning electron microscopy has revealed many new characters in gastropod radulae, especially of small species and with respect to detailed tooth morphology and function (e.g. Hickman, 1983, 1984a). These new data together with the radular features of the recently investigated groups permit a reconsideration of the taxonomic value of the gastropod radula. Whereas there is no doubt of the high diagnostic value of the radula in different gastropod groups, its value for higher systematics must be examined carefully in each case. Among the primitive Gastropoda two main radular types are present, the docoglossate and the rhipidoglossate, although there are several aberrant types which do not fit weU in any of these types (Hickman, 1983). It was mainly Thiele (1925,1929) who cemented the view that the rhipidoglossate type is the most primitive among the Gastropoda, and his opinion has been repeated up to now (e.g. Yonge, 1947; Fretter & Graham, 1962; Graham, 1985). Golikov & Starobogatov (1975) were the first modern authors, who criticized Thiele's view, and in regarding the docoglossate type as the most primitive they erected a new subclass Cyclobranchia ( = Docoglossa). Although this classification has not been accepted by other authors (see §4.1.), all recent investigators of neopilinid and docoglossate radulae agree with respect to the primitivenes of the docoglossate type (McLean, 1979; Wingstrand, 1985; Iindberg, 1986,1988). In fact, the functional type of polyplacophoran, tryblidiid, and docoglossate


391

radulae is essentially similar in that it acts as a the rhipidoglossate to the taenioglossate type of simple rasp without longitudinal bending of the radula. However, there are several aberrant radular membrane, the so-called 'stereoglos- radulae of archaeogastropods resembling supersate' condition (Salvini-Plaw6n, 1988a; Salvini- ficially a taenioglossate-like type (e.g. LepePlawen & Haszprunar, 1987). In contrast, the tellidae, see e.g. Hickman, 1983). Moreover, radular membrane of rhipidoglossate or taenio- the taenioglossate radula itself has been modiglossate radulae is longitudinally flexible fied or even reduced in various lines. Thus, ('flexoglossate' condition). Stereo- and flexo- radular conditions are of ambivalent value for glossate radula types are correlated with the higher systematics. position of the so-called horizontal muscle. In The ptenoglossate (originally ctenoglossate) stereoglossate groups this muscle has a more type has been used by several authors as a base inner-dorsal position (especially at the most to suggest systematic relationships, (i) This anterior portion of the anterior cartilage; Fig. appears correct for the Ptenoglossa ( = Jan4A), whereas it is always inserted solely at the thinoidea), because there are several other synventral outside of the cartilage in groups with apomorphic characters which support this unity flexoglossate radula. These differences also (e.g. purple gland, alimentary tract, see e.g. enable the classification of aberrant radula Thiele, 1928). (ii) Bouvier (1886, 1887) and types, as present in many of the deep-sea Robertson (1974, 1985) used the ptenoglossatearchaeogastropods (e.g. Hickman, 1983; Mar- like radula of Architectonica Roding, 1799 ( = shall, 1983a). The above mentioned large func- Solarium Lamarck, 1799) to support assumed tional difference between the two types serves as relationships between Janthinoidea and a synapomorphic character for all Gastropoda, Architectonicoidea. However, the radula of except the Docoglossa and group-C Hot-Vent Architectonica is dearly of independent origin, limpets. It thus strongly reinforces the idea that since the great majority of architectonicids (and the Docoglossa are the first gastropod offshoot. of the closely related mathildids) possess a For the differences between the docoglossate (slightly modified) taenioglossate radula and the polyplacophoran-neopilinid buccal (Thiele, 1928; Climo, 1975; Bieler, 1988). (iii) apparatus see §2.2. and §3.3. Recent inves- The affinities between Janthinoidea and Eulitigations by McLean (pers. comm.) revealed moidea are mainly based on the occurrence of that the radula of the group-C and hot-vent ptenoglossate radulae in primitive Eulimidae. limpets might reflect an intermediate condition This view should be reconsidered, since Ware'n between the docoglossate and the rhipido- (1979) described a taenioglossate radula in glossate radula. Although being still stereo- Thaleia ( = Benthonella) nisonis (Dall, 1889), glossate this type has a well-developed rhachis the shell and organization of which is otherwise tooth, non-mineralized lateral teeth, and lacks typically eulimoid. Thus, an independent origin basal plates. All these characters are present in of the ptenoglossate radula in the Eulimoidea is the rhipidoglossate radula, whereas the radulae possible, casting doubt on the postulated of Polyplacophora, Tryblidiida and Docoglossa relationships between Eulimoidea and Janhave a weakly developed rhachis tooth, minera- thinoidea (see also § 4.5.2.). lized lateral teeth and basal plates (Iindberg, Among the Stenoglossa (= Neogastropoda) 1986,1988). The latter conditions are thus primi- three distinct radula types are present, each of tive for Gastropoda. Indeed, the group-C hot- which reflects a natural (holophyletic) group vent limpets have the only type of stereoglossate (Ponder, 1973; see §4.5.4.). Starting from a radula, from which an evolution of the rhi- radula with central, two lateral and two marginal pidoglossate type appears possible. teeth, the Rhachiglossa ( = Muricoidea s.l.) Hickman (1981, 1984a) pointed out that the have reduced the marginal teeth, whereas most evolutionary advantages of an asymmetrical rhi- Toxoglossa ( = Conoidea) have retained and pidoglossate radula lies in zipper-like longi- specialized only these marginal teeth (there are tudinal folding of the teeth. However, because certain turrids which have retained the original radular asymmetry has been reached several five-teeth-radula). Finally, the Nematoglossa times independently, it is of minor importance ( = Cancellarioidea) have retained and specialized the central tooth alone. Thus, the radula for higher systematics. The taenioglossate type of radula is generally serves well as a basis for systematics among the regarded as monophyletic. The Seguenziidae Neogastropoda. show a remarkable variation in their number All allogastropod groups have a more or less of teeth (Quinn, 1984; Marshall, 1983b) and specialized or even reduced radula. The basic represent a model group for the evolution from type is taenioglossate as is still present in the

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Mathildidae (Thiele, 1928; Qimo, 1975; Bieler, 1988). It is still possible to determine lateral and marginal teeth as typical for streptoneurans, and the radular morphology characterizes each subgroup of the AUogastropoda. This is in clear contrast to the radula types of the most primitive Euthyneura (Architectibranchia: Acteonoidea, Ringjculoidea, Diaphanoidea; see Haszprunar, 1985c). Here all lateral teeth are more or less similar, and their number differs greatly even within the subgroups. A significant gap is present between the Streptoneura and Euthyneura with respect to the odontoblast, which consists of few, but large and obviously polyploid cells in the latter. 2.7.2. Buccal apparatus Aside from the radula itself, several other features of the buccal apparatus are very useful to trace interrelationships of certain groups. Although the distinct shape and structure of the jaws often characterize a group, no attempt has been made up to now to use these differences for higher systematics. Only in cases of high specialization or modification might jaw conditions reflect actual relationships. Thus, the janthinoid families are characterized by a distinct stylet apparatus (Thiele, 1928). Among the Pyramidelloidea the special variations of the jaws are useful to define subgroups (see §4.7.6.). The jaws are commonly modified to stylets (Pyramidellidae: Fretter & Graham, 1949) or completely reduced (Amathina: Ponder, 1988a) or but specialized to form a complicated jaw system (e.g. Ebala niditissima (Montagu, 1803); A. War6n: pers. comm.). Generally, proboscis formation is restricted to the apogastropod grade. Here several types have been determined from functional morphology (see e.g. Fretter & Graham, 1962:151). Whereas the pleurembolic proboscis type appears to be a synapomorphic character of the Stenoglossa ( = Neogastropoda), the systematic value of the acrembolic type has become ambiguous. This type is found in Triphoroidea, Janthinoidea, Eulimoidea, Architectonicoidea, and Pyramidelloidea and have been used to speculate about relationships between these groups (e.g. Kosuge, 1966; Robertson, 1974; Boss, 1982; Fretter & Graham, 1982; Haszprunar, 1985c, d, f). However, there are large differences between the different groups especially with respect to the position of the nerve ring and the buccal ganglia in the retracted stage. In the caenogastropod groups the buccal ganglia are situated as usual in gastropods at the

emergence point of the oesophagus. Whereas the cerebropedal nerve ring surrounds the oesophagus in the Eulimoidea (War6n, 1983), it surrounds the proboscis sheath in the Triphoroidea and Janthinoidea (Thiele, 1928; Kosuge, 1966; Climo, 1975). In the allogastropod groups the nerve ring likewise surrounds the proboscis sheath, but the buccal ganglia are often positioned more posteriorly at an oesophageal loop (Fretter & Graham, 1949; Maas, 1965; Haszprunar, 1985d, f; Ponder, 1987). Therefore the acrembolic proboscis is obviously the result of multiple convergence and cannot be used to prove relationships between the respective superfamilies. The presence of radular cartilages is clearly a primitive character of the Gastropoda (Wingstrand, 1985; Salvini-Plawdn, 1988a). In most archaeogastropod groups there are two pairs of cartilages, the anterior pair alone being interconnected by the horizontal muscle. This is probably the basic number of cartilages, which is secondarily enlarged (Patellidae, certain Neritidae, Fissurellidae) or reduced (Cocculinidae, Melanodrymia, Ncomphalus) in certain archaeogastropod groups. Radula cartilages are present in all archaeogastropods and in caenogastropods with a more or less taenioglossatelike radula for grasping. However, they are completely lacking in the Valvatidae, in the AUogastropoda, and in the Pentaganglionata. In the latter group several species are known which have so-called cartilages. These, however, differ entirely in their structure from the original cartilages and obviously are secondary structures. Although all valvatids investigated are herbivorous (Fretter & Graham, 1978), their lack of cartilages together with other modifications of their alimentary canal suggest an original carnivorous habit (Rath, 1986; see also below and 54.6.1.). The presence of a radula diverticulum is generally regarded as primitive among gastropods (Hyman, 1967, Fretter eta/., 1981). Obviously it is lost with the rhipidoglossate radula. Since it is present also in those archaeogastropods with a highly aberrant radula (several cocculiniform families; see Hickman, 1983; Haszprunar, 1988d), this supports the determination of these radulae as modified rhipidoglossate instead of taenioglossate types. Number, shape, position, and structure of the salivary glands have been used in gastropod systematics. Ponder (1973) mentioned the existence of a second (accessory) pair in the Stenoglossa, Docoglossa and certain Neritidae to suggest affinities between these groups.


However, since certain members of both archaeogastropod groups have still a single pair of salivary glands, I agree with Salvini-Plawen (1987) that the accessory pair appears of secondary origin. In addition, the unity of the Caenogastropoda can no longer be doubted (see § 4.5.). The course of the salivary ducts has also been regarded as important for caenogastropod phylogeny. However, this character varies among the Neotaenioglossa and can only be used to define groups of minor rank (e.g. Ponder, 1973, 1988b).

393

out the Mollusca (Graham, 1949; SalviniPlawen, 1981b, 1984,1988a) and of other primitive gastropod groups (Fretter & Graham, 1962) this model has only to be modified slightly, although it was based almost entirely on the conditions of zeugobranch Vetigastropoda. The large and spirally coiled caecum alone (not the caecum itself) is not accepted here as a primitive character, because this condition is not present in any of the primitive groups. It is restricted to Haliotidae, Pleurotomariidae and Trochoidea and might represent a synapomorphy of these groups (Haszprunar, 1989). The gastric shield appears to function only in combination with a rotating so-called protostyle, although the latter is not found in all groups (e.g. Cocculinidae, Pseudococculinidae with gastric shield but lacking a protostyle). This protostyle is often specialized to form a crystalline style in microherbivorous or filter-feeding Conchifera. The fate of the primitive gastropod stomach depends largely on the feeding biology of the respective group. It becomes more complicated in microherbivorous or filter-feeding species (but also in grazers such as Campanile symbolicum Iredale, 1917; Houbrick, 1981), but is secondary simplified in carnivorous or parasitic groups. Despite the value of the stomach conditions in defining certain groups, these circumstances weaken its value for higher systematics. It is generally accepted that the intestine of the first gastropods had many loops and finally penetrated the heart-ventricle. However, changes of these conditions have obviously occurred several times in different archaeogastropod lines, and again they only can be used to define single groups.

2.7.3. Oesophagus As recently outlined by Salvini-Plawen & Haszprunar (1987; Salvini-Plawen, 1988a), the conditions of the anterior oesophagus are very useful to define distinct subgroups, but can likewise be used to trace the evolutionary course of the Streptoneura in general. Aside from certain exceptions (Cocculiniformia with extreme specialized alimentary tracts; see Haszprunar, 1988d), all docoglossate and rhipidoglossate Archaeogastropoda have a dorsoventrally depressed oesophagus with so-called oesophageal pouches and a distinct pattern of longitudinal folds. These features are likewise found in the Polyplacophora, the Scaphopoda, and even in primitive (protobranch) Bivalvia; the Tryblidiida alone show somewhat modified conditions (Wingstrand, 1985; Salvini-Plawen, 1988a). This demonstrates that the oesophageal characters mentioned above are plesiomorphic for the Gastropoda. Pouches and ventral folds are lacking in all higher Gastropoda, although the Seguenziidae, Architaenioglossa and Caenogastropoda usually still have dorsal folds (so-called dorsal food channel) and oesophageal glands. This is in clear contrast to the Valva2.8. Nervous system tidae, the allogastropod groups, and the Euthyneura, where the oesophagus generally lacks folds and glands, being a simple muscular tube. The molluscan nervous system is generally accepted as a very conservative organ system and many attempts at homologizing organs between the classes are based in it. Among 2.7.4. Posterior alimentary tract all molluscan classes the Gastropoda show the In general, there is a remarkable similarity of the greatest diversity in their nervous system, and stomach among the Conchifera (Salvini-Plawen, for more than a century these differences have 1981b, 1984, 1988a). Graham (1985) charac- been used for gastropod classification. In the terized the stomach of a hypothetical gastropod following the phylogenetic value of several archetype by a major typhlosole lying to specific characteristics of the gastropod nervous the right of an intestinal groove, a sorting area system is reconsidered. extending into a spirally coiled caecum, a cuticularized gastric shield, two openings of the midgut glands, and a tubular style to the intes- 2.8.1. Streptonewy-euthyneury tine which includes also a second typhlosole. In There is no doubt that streptoneury is a the light of comparative investigations through- direct result of gastropod torsion. However, it

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should be noted that the visceral loop appears later in ontogeny than torsion (Smith, 1935; Crofts, 1937). In addition, a second difficulty is often overlooked in phylogenic discussions on torsion and streptoneury. Whereas in the Solenogastres, Polyplacophora, Tryblidiida, Bivalvia, and Scaphopoda the lateral ( = visceral) cord surrounds the dorsoventral ( = shell) muscles, this is not the case in Cephalopoda and Gastropoda, where the visceral cord (or ganglia) is situated medially of the musculature. Both phenomena can be explained by suggesting that cytochemical markers lead the outgrowing visceral cords to their final position. This does not influence the correlation between torsion and streptoneury but avoids the difficulties mentioned above. Spengel (1881) was the first to use the similarities of the nervous system of opisthobranchs and pulmonates to unite them within the taxon Euthyneura, in contrast to the remaining Streptoneura which were regarded as more primitive. This division of the Gastropoda has been weakened in the light of certain exceptions amongst the Streptoneura such as the Cingulopsidae (Fretter & Patil, 1958; Fig. 3F), Glacidorbidae, or the Pyramidelloidea (Fretter & Graham, 1949; Ponder, 1986,1987; Fig. 3H). Also among the Euthyneura several species have retained streptoneury (Acteonoidea. Ringiculoidea, several bullpmorph genera; Chilinidae, certain Elliobiidae; Fig. 3J, L), and thus the division of Spengel (1881) is not fully diagnostic. Moreover, it has been found that the euthyneurous condition can result from several, entirely different processes. As recently reviewed by the author (Haszprunar, 1985c), gastropod euthyneury can be caused by detorsion, by concentration, or by semidetorsion and concentration. To decide between the different types of euthyneury the position of the (single left) osphradium (or osphradial ganglion) is highly diagnostic, since the osphradium is included in detorsion processes. If euthyneury is caused by concentration alone, this ganglion is situated at the left of the body. This is the case in the Cingulopsidae (Fretter & Patil, 1958), in the Pyramidelloidea (Fretter & Graham, 1949; Haszprunar, 1985b; Ponder, 1988a), and in the freshwater genus Glacidorbis (pers. obs. see 5 4.7.5.) (Fig. 3F.H). In contrast, the (left) osphradial ganglion is situated at the right side in detorted animals (i.e. most opisthobranchs, all pulmonates) (Fig. 3K, M). Euthyneury by detorsion has occurred several times in evolution, since many subgroups include species which still retain streptoneury. Thus, euthy-

neury as a character is a case of multiple convergence and cannot be used by itself to trace phylogenetic relationships. However, in contrast to euthyneury itself, there is another character of the central nervous system which is diagnostic for the Euthyneura. This is the presence of an additional pair of ganglia resulting in a so-called pentaganglionate visceral loop. Although these parietal ganglia (according to Hoffmann, 1939; Regondaud et al., 1974, ='pallial' ganglia of Brace, 1977a, b, 1983) are often and variably fused with other ganglia, they are clearly visible in primitive forms (Fig. 3J-M) at least during ontogenesis. According to Brace (1977a, b) they formed by separation from the original pleural ganglia through the elongation and modification of the head. The presence of these ganglia reflects a significant gap in ecological adaptation and is therefore very useful as a basis for classification (see also §4.8. and 5.2.). It also indicates the monophyly of the included subgroups (opisthobranchs and pulmonates). 2.8.2. Hypo- and epiathroid condition Lacaze-Duthiers (1888) first mentioned the distinct differences of the gastropod nervous system with respect to the relative position of the pleural ganglia. He distinguished between 'epipodoneuran' (adjacent pleural and pedal ganglia) and 'aponotoneuran* types (adjacent pleural and cerebral ganglia). Fretter & Graham (1962) replaced these terms by 'hypoathroid' and 'epiathroid', and because these are more familiar they are used here. The author (Haszprunar, 1988c, d; Salvini-Plawen & Haszprunar, 1987) has based the classification of the streptoneurous Gastropoda on those types, and therefore they are carefully reconsidered here. Despite the great diversity of the anatomy of the different archaeogastropod groups, a hypoathroid nervous system is typical for all of them (Fig. 3A, B, D). It is likewise found in groups with a concentrated nervous system such as certain Cocculiniformia or Neritimorpha. Moreover, the hypoathroid conditions appears to be the only character which is diagnostic for the archaeogastropod grade and thus serves well as a basis for classification (see also § 5.2.). Compared with Polyplacophora and other conchiferan classes (the cephalopod nervous system alone is too concentrated to decide here unequivocally), the hypoathroid condition is likely to be an apomorphic character of the Gastropoda, being symplesiomorphically retained in the


O» °**'*r • *

o•-

Fig. 3. Comparative schematic view of gastropod nervous systems. A—Scissurellidae (Vetigastropoda). B—Neritidae (Neritimorpha). C—Viviparidae (Architaenioglossa 1). D—Ampullariidae (Architaenioglossa 2). E—Cerithiidae (Caenogastropoda 1). F—Gngulopsidae (Caenogastropoda 2). G—Mathildidae (Allogastropoda 1). H—Pyramidcllidae (Allogastropoda 2). J—Acteonidae (Opisthobranchia 1). K—Philinidae (Opisthobranchia 2). L—Chilinidae (Basommatophora 1). M—Lymnaeidae (Basommatophora 2). Abbreviations: ac—accessory ganglion; ce+t+r—cerebral ganglion with tentacular and rhinophoral nerve; ns—neurosecretory centres (procerebrum, cerebral gland, dorsal body); os—osphradial ganglion; p+1—pedal ganglion with lateral nerve; pa—parietal ganglion; pi—pleura] ganglion; sb/sp—sub-rcsp. supraoesophageal ganglion; v—visceral ganglion.

Archaeogastropoda (Salvini-Plawe'n & Haszprunar, 1987). The hypoathroid condition is also typically present in the architaenioglossate groups Cyclophoroidea and Ampullariidae. The Viviparidae have a so-called dystenoid nervous system, which according to Fretter & Graham (1962: 308/309, fig. 161) should be represented by well-separated but still ventrally situated pleura! ganglia. However, original investigations on the viviparid nervous system (Bouvier, 1887; Anandale & Sewell, 1921; Stannuhlner, 1974, 1983; pers. obs.) clearly show that the viviparid nervous system is hypoathroid on the left side and epiathroid on the right side (Fig. 3C). Since the Ampullariidae have a typical hypoathroid nervous system (Bouvier, 1887; Stannuhlner, 1969; Honegger, 1974; Fig. 3D), the viviparid

condition is probably an independently evolved secondary condition. If the Ampullarioidea are a grade, however, then the dystenoid type of the Viviparidae (the term should be restricted to these unique conditions) might be a step towards the epiathroid condition. In any case the archaeogastropod grade (including the architaenioglossate groups) can be unequivocally defined by a 'streptoneurous and hypoathroid or dystenoid nervous system' (see SarviniPlawdn & Haszprunar, 1987 and also § 5.2.). The idea that the dystenoid type is an independent (intermediate or secondary) condition is supported by a second character which is correlated with the hypoathroid/epiathroid distinction. Whereas all archaeogastropods (s.l.) have simple tentacular nerves, these are basally bifurcated in those gastropods which

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have the epiathroid type (there are certain exceptions in very small species). Also Viviparidae have simple tentacular nerves, reflecting their archaeogastropod nature. The functional reason for the change from the hypoathroid to the epiathroid (or dystenoid) type is not clear. According to Fretter & Graham (1962) the snout-formation of the head and the position of the cerebral ganglia posterior to the (somewhat reduced) buccal mass enabled this change. In fact, the dystenoid type of the snout-bearing Viviparidae in contrast to the hypoathroid type in the snout-less Ampullariidae supports this theory. However, the evolutionary advantage of the change itself is still unknown. The true epiathroid type has probably evolved only once. It is present in the Caenogastropoda, Valvatidae, Allogastropoda and in primitive Pentaganglionata, reflecting the monophyletic origin of these groups (Fig. 3E-J). As mentioned above, all these groups with a 'streptoneurous (i.e. left-side osphradial ganglion), triganglionate (i.e. lack of parietal ganglia), and epiathroid nervous system with bifurcated tentacular nerves' can be united in a distinct grade called Apogastropoda (Salvini-Plawe'n & Haszprunar, 1987; Haszprunar, 1988c). Among the Euthyneura several groups have an hypoathroid-like nervous system (Aplysiomorpha, Gymnosomata, Ellobiidae, Trimusculidae, Stylommatophora; Lacaze-Duthiers, 1888). However, in all these cases it is (primarily) only the anterior portion of the original pleural ganglion (the posterior portion is the parietal ganglion, see above) which changes its relative position. Thus, these conditions are not directly comparable with those of the Archaeogastropoda. Nevertheless, these changes might be very useful in tracing interrelationships of the pentaganglionate subgroups.

ziidae, Viviparidae). Thus, its value for higher systematics is low. (3) The posterior migration of the cerebral ganglia behind the buccal mass is a general trend among the Caenogastropoda. Because objective measurement of this character is very difficult especially in less concentrated nervous systems, where it depends largely on the degree of contraction of the animal, it has been never used in a phylogenetic discussion. The position of the nerve ring with respect to a proboscis, however, is useful to distinguish different proboscis types (§2.7.2.). (4,5,6) Reduction of the labial commissure and of the labial ganglia results in the buccal ganglia alone supplying the buccal apparatus. This trend occurs also in several archaeogastropod groups especially in deep-sea or hotvent forms (Cocculiniformia, Melanodrymia, Seguenziidae). Therefore this character cannot be used to speculate about phylogenetic relationships despite its defining value. (7,8) The change from the hypoathroid to the epiathroid type is discussed above. (9,10,11) The concentration of the pedal nervous system, resulting in true pedal ganglia and in a single (or two) pedal commissures is also found in certain archaeogastropod groups such as the Cocculinidae or Cocculinellidae (Haszprunar, 1987c, 1988a). Moreover, retention of pedal cords in certain Caenogastropoda (Cerithioidea-Lau/geria; Cypraeoidea) makes the convergence of this concentration obvious. Again it cannot be used for higher systematics. (12) For the position of the statocysts and their nerves and for the phylogenetic value of the subradular organ/ganglia see §2.9.6. and 2.9.7). Although dialyneury and zygoneury have been attracted much attention from the early anatomists (e.g. Bouvier, 1887; Pelseneer, 1899), they are not used in phylogenetic analysis by modern authors, despite the diagnostic value of these characters. Conditions of the visceral loop vary among the Gastropoda to such a high 2.8.3. Other characters of nervous system degree that they can only be used for phyloFretter & Graham (1962: 308) listed 12 main genetics in cases of high specialization (e.g. trends in the evolution of the nervous system distinct fusion of ganglia such as in the Neriamong the streptoneurous Gastropoda. Several timorpha; Fig. 3B). Also the visceral loop is of these trends are directly correlated with each primitively represented by a neural cord (no other, whereas some more can be added in the distinct ganglia). Concentration to form distinct visceral ganglia has occurred gradually and sevelight of recent investigations. (1,2) The dorsal shift of the cerebral ganglia ral times, and thus this character is also of little and thus the shortening of the cerebral com- phylogenetic value. missure is clearly an advanced feature, but has Recent investigations of the nervous systen of been independently evolved in several archaeo- the Pyramidellidae (Huber, 1987) revealed two gastropod lines (e.g. Cocculinellidae, Seguen- interesting nerves on which phylogenetic con-


sideration can be based. These are the so-called rhinophoral nerve (because of its homology with that of the opisthobranchs) of the cerebral ganglion, and the so-called lateral nerve, which emerges immediately beneath the pleuropedal connective at each pedal ganglion and possibly also contains pleura! fibres (Fig. 3H). These nerves are not present in other allogastropod groups (Huber, 1987) but both are typical of opisthobranchs, and the lateral nerve is also present in pulmonates (Fig. 3J-M). This distribution strongly suggests that the Pyramidelloidea represent the last offshoot before the euthyneuran level has been attained. There is no evidence supporting the view that Pyramidelloidea are degenerate opisthobranchs (see e.g. Gosliner, 1981; Haszprunar, 1985c).

2.9. Sense organs Comparative fine-structural investigations of various sense organs of the Gastropoda have revealed new data which appear very useful for tracing phylogenetic relationships. However, it should be stressed that only fine-structural characters of high complexity or specialization can be used in this respect, since the danger of analogous characters is very great in finestructural research (Rieger & Tyler, 1977, 1985). As an example, the mechano-sensitive collar receptors exhibit a nearly identical and highly specialized ultrastructure, although they occur in clearly analogous sense organs (Haszprunar, 1985e). Therefore great care and large comparative series throughout the whole group are necessary to distinguish between homology and analogy. 2.9.1. Cephalic tentacles The fine-structure of the cephalic tentacles of several streptoneurous gastropods have been investigated (Storch & Welsch, 1969; Storch, 1972; Crisp, 1981; Stfitzel, 1984). However, as yet no attempt has been made to use these data for phylogenetic consideration, although there are considerable differences in the fine-structure of cephalic tentacles between the groups. Thus, most Vetigastropoda and Seguenzioidea are characterized by so-called 'brush-organs', representing tentacles which are composed of numerous papillae (e.g. Storch, 1972; Crisp, 1981). The Caenogastropoda include several distinct types which may be useful to trace their still ambiguous (see §4.5.) interrelationships.

397

For instance, the cephalic tentacles of Aporrhaidae and Cypraeidae share a very similar and distinctive vacuolized epithelium (Storch & Welsch, 1969; Storch, 1972). Additional investigations throughout the Streptoneura are necessary to ascertain the phylogenetic value of such characters. For the use of cephalic tentacles as copulatory organs see §2.6.1. 2.9.2. Epipodium The presence of epipodial tentacles is generally regarded as a primitive character. Because of their pedal innervation, they represent a specialized portion of the gastropod foot. Interestingly enough, they often have the same shape and structure as the cephalic tentacles, although the latter are cerebrally innervated—a typical case of so-called normative homology (see Haszprunar, 1985e for a discussion of this phenomenon). Similarly, the sensory papillae of the Vetigastropoda occur on (cerebral) cephalic tentacles, (pedal) epipodial tentacles, and around the (pleural) mantle slit/hole (Crisp, 1981; Herbert, pers. comm.). According to the conditions in the Vetigastropoda (in particular Scissurellidae, Trochoidea), the gastropod archetype should have several pairs of such tentacles starting behind the head along the lateral foot of the animal (Graham, 1985). However, there are symmetrical limpets with a single posterior pair of epipodial tentacles (many species of CbccuJiniformia). More convincingly this condition is also found in early juveniles of the Docoglossa, Fissurelloidea and Scissurellidae, the adults of which respectively lack epipodial tentacles and have many of them (e.g. Anderson, 1965; McLean, 1984; Haszprunar, 1989). Therefore it is much more likely that the presence of a single posterior pair of epipodial tentacles is the most primitive gastropod condition. Secondary elaboration of the epipodium obviously occurred several times in different archaeogastropod lines, probably correlated with higher mobility. This can be used to distinguish primary ( = symmetrical) and secondary ( = asymmetrical) archaeogastropod limpets. The former have always had low mobility and thus have none (adult Docoglossa; Group-C hot-vent limpets; several Cocculiniformia) or a single posterior pair (juvenile Docoglossa; many Cocculiniformia) of epipodial tentacles. By contrast, secondary archaeogastropod limpets had coiled and rather mobile ancestors and thus have several pairs of epipodial tentacles

398

G. HASZPRUNAR

(Lepetodriloidea, Neomphalus, fissurelloid and scissurelloid limpets). Also certain Caenogastropoda (Eatoniellidae, Lacuninae, Litiopidae) possess epipodial tentacles, reflecting their relative primitiveness. Whereas epipodial tentacles are present in many archaic groups, so-called (e.g. Crisp, 1981) epipodial sense organs (ESO) are restricted to the Vetigastropoda. Although not yet confirmed in the Pleurotomariidae, the ESO were regarded as a synapomorphic character of this assemblage • by•Salvini-Plawe'n & Haszprunar (1987). Because ESO are likewise present in the Lepetodriloidea (pers. observ. on two species), the latter group should be included in the Vetigastropoda (see § 4.3.). Whereas in Lepetodriloidea and in many ScissureUoidea only a single ESO-tentacle is present, ESO and epipodial tentacles are correlated in all other vetigastropod groups (Haszprunar, 1989).

2.9.4. Osphradium

Comparative fine-structural investigations on the gastropod osphradium (Haszprunar, 1985a, b) have resulted in very useful data that can be utilized in phylogenetic analysis of the streptoneurous groups. Since species of related groups, but of very different habitats (e.g. marine, freshwater, terrestrial) and habits (e.g. herbivorous, carnivorous, filter-feeders, parasites), were investigated, it was possible to distinguish between ecological adaptation (size and shape of the osphradium) and systematically correlated characters (cell types). Meanwhile it has become obvious that the phylogenetic significance of the osphradial data is even higher in certain cases than originally assumed. (1) The Docoglossa have an osphradium, the structure of which differs from those of all other gastropods investigated in lacking true sensory cells (see also Stfltzel, 1984). Moreover, these osphradia are situated on the floor of the mantle cavity (Fig. 1A), whereas the osphradia of all 2.9.3. Eyes other Gastropoda lie at the mantle roof. The Most archaeogastropod groups have their eyes original statement, that 'they (Docoglossa) situated at the outer base of their cephalic appear as a very isolated group' (Haszprunar, tentacles. Vetigastropoda alone have evolved 1985a), is now confirmed by several other argudistinct eye-stalks, and again this should be ments (see §4.1.1.). (2) The close relationship regarded as an apomorphic specialization rather between Haliotoidea and Trochoidea can be than a primitive condition. The Rissoellidae supported by anatomical characters (see Salvinipresent a considerable variation of their exact Plawdn & Haszprunar, 1987 and §4.3.1.). The eye-position with respect to their tentacle base common position of the osphradium at the free (Ponder & Yoo, 1977), and also the eyes of efferent ctenidial axis supports the phylogenetic Glacidorbis are placed behind the tentacular unity of the Vetigastropoda. (3) The entirely base (Ponder, 1986). The eyes of the Pyra- different osphradial types of archi- and neomidclloidea have migrated towards the mid- taenioglossate groups parallels their different line, resulting in a position between the types of nervous system (see above). (4) The tentacles. This is also typical for the primitive monophyly of the Caenogastropoda (s. str.) is opisthobranchs and again reflects the affinities based on the constant mutual position of three special osphradial cell-types. (5) The osphradial between these groups. structure of Campanile symbolicum differs from As has long been known the Docoglossa have the caenogastropod and the allogastropod type, the most primitive eye-type of all gastropods. It reflecting its isolated position (see § 4.5.1.). (6) lacks a lens, which is present in the eyes of all The exclusion of Valvatidae from the Caenoother gastropods (Fig. 4E). Once more, this gastropoda cannot longer be doubted (Rath, primitive eye-type reflects the basic position of 1986, 1988; §4.6.1.). (7) There is increasing the docoglossate offshoot. The Vetigastropoda evidence that the Allogastropoda with an opisare also characterized by a distinct eye-type, thobranch-like osphradia] type represent an which is equipped with a lens but is typically an intermediate grade between typical proso- and open vesicle, the Scissurellidae and FissureUidae opisthobranchs (see §4.7.). having closed eye-vesicles. Both types are plesiomorphic, however, and a closed eye with lens occurs also in several other archaeogastropod groups, indicating a multiple origin of this state. 2.9.5. Bursicles and similar structures Among the Caenogastropoda, the Heteropoda Although ctenidial sense organs have been alone can be characterized by their (synapo- known for several decades (Bourne, 1910; Hart, morphic) telescopic eyes (Fretter & Graham, 1937; Szal, 1971), their phylogenetic significance 1962: 318). has been considered only recently (Haszprunar,


1987b). They are present in all vetigastropod species investigated, even in those with specialized and modified ctenidial leaflets such as the filter-feeding trochid genera Umbonium or Lirularia (Mclean, 1986). They are also present in the vetigastropod groups that inhabit hydrothermal vents Haszprunar, 1989; pers. obs.). Neomphalus lacks bursicles (Fretter, pers. coram.; pers. observ.), though it also has bipectinate ctenidia with skeletal rods. Because the fine-structure of the bursicles of all major vetigastropod groups is identical (no fine-structural data on Lepetodriloidea), they were regarded as Haszprunar (1987c, Salvini-Plawdn & Haszprunar, 1987) as a synapomorphic character of the Vetigastropoda. Similar (sensory?) pockets are present in the gill leaflets of primitive lepetelloid families (Haszprunar, 1988b, d, McLean & Haszprunar, 1987). However, all these leaflets are clearly of secondary origin, thus their pockets cannot be directly homologized with those of the Vetigastropoda. 2.9.6. Subradular organ Because a subradular organ is present in Polyplacophora and Tryblidiida, its occurrence among primitive gastropods must be regarded as plesiomorphic. In addition, there are many archaeogastropod groups which have secondarily lost this organ, especially those from deepsea or hot-vent-habitats (e.g. Cocculiniformia, Melanodrymia, Neomphalus, Seguenziidae). Once more this character is quite useful to define groups but cannot be used to trace their interrelationships. 2.9.7. Statocysts Statocysts are typical of all conchiferan classes (Wingstrand, 1985). The homology of the neopilinid statocysts with those of the other classes has been doubted on the basis of their lateral position, instead of a more or less pedal one in the other classes, and because their cerebral innervation is not yet confirmed. However, because such a lateral position is also found in the Docoglossa (Fig. 4F), this appears now to be simply the primitive condition. In addition, all statocysts originate as lateral ectodermal invaginations (several bivalves retain open ducts), thus an original lateral position is more probable than a pedal one. The retention of the lateral position of the statocysts only in the Docoglossa once more reflects the basal position of this offshoot.

399

It is frequently assumed that the presence of several small statocones is a primitive character, whereas a single, concentrically structured statolith represents the advanced (apomorphic) condition. Though the former condition is present in Neopilinidae and Docoglossa, there are several archaeogastropod groups with statoliths. Moreover, many caenogastropods and opisthobranchs are known with the statocysts filled by many tiny statocones. Thus this character is highly ambiguous and should only be used for definition of groups. 2.10. General conclusions Table 2 summarizes the arguments presented so far with respect to homology or analogy, and apomorphy or plesiomorphy of the various organs and characters. There are only a few characters for which the traditional plesiomorphic or apomorphic status should be reconsidered (e.g. symmetrical limpet-like shell, docoglossate radula type, acmaeid ctenidial type, simple epipodial formation). This is primarily caused by the now well-founded assumption that the Docoglossa, and not the Pleurotomarioidea (sensu lato), are the most primitive offshoot of the gastropod line. In many cases the primitive state of a character is quite clear, but several convergent lines towards the advanced state can be detected (e.g. loss of right shell muscle, loss of right kidney, proboscis, internal fertilization, concentration of pedal cords, euthyneury). Detailed studies on the characters themselves (e.g. by fine-structural methods), on their mode of development, or on their function appear necessary to decide definitively each case of convergence.

3. ON THE ORIGIN OF GASTROPODA 3.1. Gastropoda as Mollusca 3.1.1. The aculiferan groups There is large (but no complete) agreement among modern neontologists about the most probable course of molluscan evolution (e.g. Stasek, 1972; Haas, 1982; Lauterbach, 1984; Wingstrand, 1985; Salvini-Plaw6n, 1984, 1985; Scheltema, 1988). Generally the aplacophoran classes are regarded as representing the primitive level of the MolJusca, followed by the Poly-

absent coiled asymmetrically limpet-like helicoid hyperstrophic/heterostrophy secondary larval shell one (left) solid also at shell wall deep hypertorted detorted (broad sense) with pallial caecum one (left) with long efferent membrane with skeletal rods monopectinate absent/replaced present one (functional) auricle one (left) pallial blood supply internal fertilization true gonoducts closed pallial gonoducts gonoduct on pallial floor with penis pleural/pedal penis rectractile penis euspermatozoa only with apical bleb with apical vesicle with paracrystalline connected by chalazae planktotrophic with PMO (larval kidney)

present symmetrically limpet-like coiled symmetrically coiled orthostrophic primary larval shell only two (left and right) distinct bundles at spindle alone shallow torted torted lacks pallial caecum two (left and right) without long membranes without skeletal rods bipectinate present absent two (functional) auricles two (left and right) visceral blood supply free fertilization release through right kidney open pallial gonoducts gonoduct on pallial roof aphallate (spermatophores) cerebral penis contractile penis eu- and paraspermatozoa acrosome simple acrosome simle lacking paracrystalline non-connected non-planktotrophic no PMO (larval kidney)

SHELL

EGGS LARVAE

SPERM

GENITAL SYSTEM

CILIARY TRACTS HEART KIDNEY(S)

CTENIDIA

MANTLE CAVITY

SHELL MUSCLE(S)

PROTOCONCH

advanced (apomorphic) state

primitive (plesiomorphic) stage

ORGAN

one one one one two one

(43) (39) (43) (39) (11, 29) (39)

one (39)

several (4, 11, 13, 15, 17, 23, 24) one (10) one (14) several (18p, 19p, 23p, 24) several (3, 4p, 6, 9, 30p, 37) one (39) several (2, 7, 11, 13, 15, 24) several (8, 11, 13, 15, 26) one (39) many several (8, 9p, 11, 13, 15, 25, 26) many (27, 28, 31 pp, 38, 41 p, 42, 43) one (45) many many (31pp) one (45)

one (49)

many (11p, 31pp) two (9p, 10) many two (10, 29) one (39) two (11, 29) many two (9p, 10) one (49) one 110) two (15, 31p) many (31pp)

number of changes

Table 2. Review of the character analysis. The numbers in brackets correspond with the phylogram (Rg. 5). p = in part.

O

BURSICLOIDS SUBRADULAR ORGAN STATOCYSTS

OSPHRADIUM

EYES

EPIPODIUM

VISCERAL LOOP

INTESTINE RECTUM ANTERIOR NERVE-RING

STOMACH

JAWS CARTILAGES DIVERTICLE SALIVARY GLAND OESOPHAGUS

RADULA

stereo- (doco-) glossate rhipidoglossate present present present pouch-like with pouches pouches non-papillate with dorsal food channel with gastric shield with (simple) caecum sorting areas with several loops penetrates ventricle hypoathroid with labial commissure pedal cords simple tentacular nerve no rhinophore nerve no lateral nerve cord-like streptoneury no parietal ganglia one pair of tentacles lacking ESO present lacking lens open vesicle lateral positition two not zoned beneath ctenidial axis lacking sensory cells lacking SM/SI2/SI4 absent present lateral statocones

flexoglossate taenioglossate absent absent (radula present) absent with duct lacks pouches pouches papillate lacks dorsal food channel lacking gastric shield lacking caecum lacking sorting area with few loops passes ventricle epiathroid lack of labial commissure pedal ganglia bifurcated tentacular nerve with rhinophore nerve with lateral nerve visceral ganglia euthyneury with parietal ganglia several pairs of tentacles with ESO lacking with lens closed vesicle median position one (left) zoned at ctenidial axis with sensory cells with Sii/Si2/Si4 present absent ventral statolithes

one (5) one (26) many one (37) one (24) several (2, 9p, 26) two Op, 24) one (16) several (31 pp, 37) several (9p, 31 pp, 37) several (2, 9p, 26) several (2, 9p, 37) several (9p, 31 pp, 37) several (2, 8, 9p, 13, 15,24) one (29) several (7, 13, 15, 24) several (8p, 9p, 30p, 35) one (29) one (47) one (47) several (8p, 9p, 16pp, 24) several (31 pp, 46, 48, 49pp) one (49) one ? (12) one (16) many one (5) several (8p, 9p, 11, 18, 19, 26) one (47) several (4p, 7, 11, 13, 15, 23, 24) several (11, 18,26) one (16) one (5) one (30) several (9p, 13, 16) several (7, 11, 13,24) one (5) several (8, 13, 15, 31 pp, 49pp)

VI

D

3

o

3

o >

o

f

o

m

§ s o 2 ]•

402

G. HASZPRUNAR

placophora and finally the Conchifera. By contrast, certain paleontologists (e.g. Runnegar & Pojeta, 1985) still hold the opposite view in regarding monoplacophoren Conchifera as the most primitive molluscs. It is argued that the fossil record of the Polyplacophora starts much later than that of the Conchifera. However, in the light of recent findings of tiny Polyplacophora from the lowest Cambrium (Yu, 1987), this argument must be abandoned. In addition, cladistic analysis shows that the Polyplacophora represent an intermediate offshoot between the aplacophorous and the conchiferous level of evolution (see below). Salvini-Plawen (1980, 1981a, 1984, 1985) elaborated the view that the Caudofoveata represent the first offshoot of the moUuscan line. However, in the light of the still unknown ontogeny of the Caudofoveata, the assumed synapomorphies for the Solenogastres, Polyplacophora and Conchifera (Adenopoda: Foot sole restricted to the pedally innervated portion, with pedal gland, primitively seven middorsal transverse rows of juxtaposed scales in ontogeny) might have already been present in the molluscan archetype (see also Scheltema, 1988). If so, the (cerebrally innervated) 'pedal' shield of the Caudofoveata would represent a highly specialized portion of the anterior body and not a restricted original glide-sole as assumed by Salvini-Plawen. On the other hand, all similarities between the Caudofoveata and the Solenogastres are either based on plesiomorphies (cuticularized mantle with scales, muscle system, nervous system, simple radula) or on convergences (worm-like shape, mid-ventral narrowing of the pedal sole, gonopericardial system). Thus, the exact position of the apla-

cophoran classes relative to each other is not yet fully ascertained (see expression of this uncertainty in Table 3). In contrast, the monophyly of the Polyplacophora and Conchifera (sometimes united as Testaria) is indicated by several synapomorphic characters (Salvini-Plawen, 1981a; Haas, 1982). The polyplacophoran shell plates consist primarily of three layers as do the conchiferan shells; there are (twice) eight pairs of dorso-ventral muscles as in the primitive Tryblidiida, true excretory organs ( = kidneys) are formed by specialization of the pericardial ducts in both groups, and the polyplacophoran alimentary tract (with a broad, stereoglossate radula, subradular organ, oesophageal structure, multi-looped intestine) strongly resembles primary conchiferan conditions (except the stomach, see below). Many of these apomorphies are retained in the Gastropoda, especially in the Docoglossa (see §4.1.).

3.1.2. Monophyly of Conchifera The monophyletic origin of the conchiferan classes among the Mollusca is accepted by nearly all modern authorities. It is also generally believed that the Conchifera form a 'crown group' of the Mollusca, whereas the remaining classes represent the 'stem group' Aculifera, an orthophyletic grade (see §5.1.). Recently Wingstrand (1985) listed 11 synapomorphies of the conchiferan classes. Three groups of characters can be determined. The presence of a single shell, with primarily identical structure (three layers) and mode of development (shell gland, embryonic

Table 3. Clado-evolutionary classification of Recent molluscan classes. Notes: s.m. = sedis mutabilis sensu Wiley (1981), marking an uncertain exact position of a subgroup. A *Taxon* is an orthophyletic group (see Table 4). Phylum MOLLUSCA Subphylum "Aculifera* Class CAUDOFOVEATA (s.m.) Class SOLENOGASTRES (s.m.) Class POLYPLACOPHORA (-PLACOPHORA) Subphylum Conchifera Superclass *Monoplacophora* Class TRYBLIDIIDA (s.m.) Superclass Cyrtosoma (= Rhacopoda, Visceroconcha) Class GASTROPODA Class CEPHALOPODA (=SIPHONOPODA) Superclass Diasoma (= Lobopoda, Loboconcha) Class SCAPHOPODA (= SOLENOCONCHA) Class BIVALVIA (= PELECYPODA)


shell) is clearly the most obvious synapomorphic condition in the conchiferan classes. It corresponds also with a new type of the pallial margin (three parallel folds) and with the characteristic location of the periostracal gland (Haas, 1981). The second group of conchiferan synapomorphies concerns the alimentary tract which is commonly equipped with anterior jaws. In agreement with Salvini-Plawen (1981b, 1984, 1985, 1987) Wingstrand (1985) regards the presence of a so-called protostyle ( = primitive crystalline style) as a synapomorphic character of the Conchifera and thus not present in the polyplacophoran ancestor. This is in some contrast to the fact that the alimentary tract as a whole is essentially similar in the Polyplacophora and in primitive Conchifera (radular type, subradular organ, hollow cartilages, oesophageal type, several loops of the intestine). The functional cause of the major change of the stomach conditions remains obscure. The third group of synapomorphic characters of the Conchifera comprises the nervous system with a sub- instead of suprarectal commissure. In addition, the presence of (probably homologous, see § 2.9.7.) statocysts and of preoral (i.e. cerebrally innervated) appendages is regarded as synapomorphic for the Conchifera. The latter character is believed by many authors to be restricted to Recent Tryblidiida (velum) and Gastropoda (cephalic tentacles). However, as stressed by Salvini-Plawen (1980, 1981a, 1984) and recently confirmed by Budelmann & Young (1985), the 'arms' of the Cephalopoda are likewise cerebrally innervated (die respective nerves pass the pedal ganglion without synaptic contacts) like the captaculae of the Scaphopoda and the oral lappets of the Bivalvia. Thus, cerebral sense organs are present in all conchiferan classes and are regarded as homologous organs.

403

ted to the Recent forms (see also below). The number of autapomorphies of the tryblidiid (neopilinid) offshoot is low but sufficient. The most obvious one is the enormous elaboration of the oesophageal pouches forming large cavities, which were originally erroneously described as a 'dorsal coelom' (Lemche & Wingstrand, 1959). In addition, the neopilinid stomach appears to be somewhat modified and specialized (Salvini-Plawen, 1981b, 1984, 1988a). Secondly, the neopilinid ctenidia are modified in being monopectinate, though vestigial leaflets of the other side are still present in Neopilina galathea (Lemche & Wingstrand, 1959; SalviniPlawen, 1981a). Thirdly, the osphradia have been lost. To interpret the conditions of the neopilinid genital and renopericardial system is more difficult and ambivalent. On the one hand, the serial arrangement of ctenidia and the heart with two pairs of auricles resemble those of the Polyplacophora and is often regarded as homologous. Consequently it is assumed that the low number of ctenidia and excretory openings in the remaining conchiferan classes is a secondary condition. However, this view is contradicted by the conditions in the most primitive Bivalvia, in particular Nuculida, where only a single pair of ctenidia and excretory openings, but a peripedal mantle cavity is present. No reason can be given for a reduction of the number of ctenidia and excretory openings. This suggests that the first bivalves did not undergo a stage in which the mantle cavity was more restricted. Thus, it is more likely that a single pair of pallial organs was originally present in the Mollusca, still retained in the Caudofoveata, Diasoma and Gastropoda, but independently multiplied in Polyplacophora. Tryblidiida, and Nautilus (Salvini-Plawen, 1980, 1981a, 1984, 1985). This view is supported by clear differences in the mode of multiplication in the Polyplacophora, which have a single pair of kidney openings and osphradia, but a variable 3.1.3. Tryblidiida* number of ctenidia due to different size (Hunter Modern authors largely agree in regarding the & Brown, 1965). Moreover, judged from the Neopilinidae as the earliest extant offshoot of relative position of ctenidia and excretory and the conchiferan stock, but no longer as an genital openings, it is likely that the multiarchetype of the Conchifera (Salvini-Plawdn, plication of the ctendia occurred twice in the 1984; Wingstrand, 1985). Difficulties arise, how- Polyplacophora, once backwards (Lepidochever, if fossil representatives, where only shell itonida), once forwards (Chitonida and and muscle characters are available, are Acanthochitonida). In contrast, Recent Tryincluded in the discussion, which is here restric- blidiida and Nautilus have correlated the number of excretory openings with the number of ctenidia. Consequently, I regard the serial * In accordance with Winfttrand (1985) mod SarvmJ-Pbwen (1985, arrangement of ctenidia and kidneys, the rela1987) I me thfruxon if • uaiiow dcftmtioo a warranted Tbe tradhioaal m o o Monoplacopbora a used as • descriptive term for cart? tive positions of which are not correlated with coochiferarjs.

404

G. HASZPRUNAR

the serial arrangement of the shell muscles and neural connectives (see Wingstrand, 1985), as apomorphic for the neopilinid offshoot, possibly to serve increased metabolism as assumed by Salvini-Plawen (1985). However, for a definitive interpretation of all these characters ontogenetic studies are necessary. If one regards the Neopilinidae as the first extant conchiferan offshoot, the question arises of the monophyly of the remaining classes. Lauterbach (1983,1984) has created the taxon Ganglioneura to unite all higher conchiferan classes. However, primitive Gastropoda (especially Docoglossa, Vetigastropoda), Bivalvia (Nucula; see Haszprunar, 1985e), Scaphopoda (pers. obs.), and Cephalopoda (Nautilus; see Young, 1965) have more or less retained the original cord-like nervous system. This clearly demonstrates that the development of true ganglia has happened within the different classes and thus cannot be used to trace interrelationships among the Conchifera. Wingstrand (1985) observed the common presence of cartilages with large, hollow vesicles and of the thick paired musculus radxdae longus (m.ra.l.), which is dorsally attached to the shell (plate), in the Polyplacophora and Tryblidiida. In contrast, Cephalopoda and Gastropoda possess cartilages composed of many, rather small, vacuolated cells and lack the radular retractors as do the Scaphopoda. Since the scars of these muscles are quite characteristic in position (anteriorly median) and elaboration (often with a mottled appearance), they can help to classify fossil forms, because this muscle is absent in all other conchiferous classes. The loss of the m.ra.l. and the structural change of the cartilages cannot be explained by differences in the radula itself, because the Lepetidae have a nearly identical radular type to the Neopilinidae and certain chitons (Wingstrand, 1985; SalviniPlawen, 1988a). The filled type of cartilages and the lack of shell-inserted radular retractors might in fact be synapomorphies of all higher conchiferan classes. On the other hand, the cartilages of the Scaphopoda are composed of very few, large cells, and their buccal apparatus is highly specialized (Salvini-Plaw6n, 1988a). The Bivalvia lack the buccal apparatus entirely. Therefore convergence of the loss of hollow vesicles and the m.ra.l. cannot be excluded. 3.1.4. Diasoma (—Ancyropoda, Loboconcha) The close relationship between the Scaphopoda and the Bivalvia is generally accepted today.

With the inclusion of the somewhat interconnecting ('pseudobivalved') fossil class Rostroconcha the taxon Diasoma was originally proposed by Runnegar & Pojeta (1974) and general agreement is found among modern authorities about the groups included (Pojeta & Runnegar, 1985; Salvini-Plawen, 1980, 1981a, 1984, 1985; Wingstrand, 1985). The similar mode of shell formation (Loboconcha; SalviniPlawen, 1985), the similar type of foot (Ancyropoda; Lauterbach, 1983), and the (synapomorphic) differentiation of the nervous system (epiathroid, true pedal ganglia, visceral ganglia with identical position and innervation areas) serve as synapomorphies of the Diasoma. In addition, the recent diasomate classes present an ancestral condition in having retained a peripedal mantle cavity corresponding with a peripedal visceral cord. Most recently, Gustafson (1987) claimed doubts on the monophyly of Protobranchia and the rest of Bivalvia ( = Autobranchia after Salvini-Plaw6n 1980), because the protobranch larvae (a pericalymma = test cell larva) is more similar to that of the Solenogastres than to that of the Autobranchia (primary with planktotrophic Veliconchia). This view must be rejected, because it neglects all autapomorphies of Bivalvia and all synapomorphies of Diasoma. Moreover, also in the Gastropoda planktotrophy is a secondary phenomenon which has been evolved at least twice (Neritimorpha, Apogastropoda and Euthyneura; see §2.6.5.), so there is no reason to reject secondary planktotrophy in the Bivalvia. 3.1.5. Cyrtosoma (s. str = Rhacopoda, Visceroconcha) Paleontologists in particular have suggested a common origin of the Cephalopoda and Gastropoda. Runnegar & Pojeta (1974) proposed the taxon Cyrtosoma to unite both classes together with the Tryblidiida. This taxon has been restricted by Wingstrand (1985) to include Cephalopoda and Gastropoda only, because no actual synapomorphy of the originally proposed assemblage exists. However, certain fossil monoplacophoran groups (those with assumed restricted mantle cavity and loss of the m.ra.l.) may be included. Other authors agree with this view (Salvini-Plawen, 1980, 1981a, 1984, 1985; Lauterbach, 1983, 1984). Indeed, several characters are shared by the two classes. First, the mantle cavity of Cephalopoda and Gastropoda is restricted to the posterior (anterior after torsion) end of the body.


This restriction of the primary peripedal mantle cavity is correlated with the visceral nerve cord which no longer surrounds the dorso-ventral (shell) musculature, but is situated between or in front of them. This permits a much higher degree of concentration of the nervous system. The second shared character of Cephalopoda and Gastropoda is the presence of a free head and the restriction of the mantle/shell to the visceral part of the body (Visceroconcha; Salvini-Plawen, 1985). Probably correlated with this character, cerebrally innervated eyes exist in the adults (Salvini-Plawen & Mayr, 1977). The presence of lateral ocelli in the juveniles of Polyplacophora and pteriomorph Bivalvia seems to weaken this argument. In the Polyplacophora each ocellus-nerve emerges at the most anterior portion of the lateral cord, but the position of the synaptic contact is still unknown (Heath, 1904; Salvini-Plawen, pers. comm.). Therefore one might think that cerebral eyes in juveniles might be a common character of Polyplacophora and Conchifera. However, the lack of such ocelli in the early larvae of all primitive gastropods and the total lack of eyes in the most basal Bivalvia contradict this view. It is more likely that the larval ocelli of chitons and pteriomorph Bivalvia are specific adaptations of the respective groups, and that the cerebral eyes represent a synapomorphic character of the Cyrtosoma. A third common character of Cephalopoda and Gastropoda has been recently elaborated by Brown & Trueman (1982; Trueman & Brown, 1985a, b). Whereas most molluscs extend their various appendages by lymph pressure alone, the Cyrtosoma ( = Rhacopoda; see Lauterbach, 1983) possess an additional specific muscle system to do so. This muscle system resembles that of the vertebrate tongue and allows much faster

405

extension of the foot, tentacles or other processes. Summing up, sufficient evidence exists for a monophyletic origin of the Cyrtosoma. The cephalopod offshoot is quite clearly characterized by the chambered shell which serves also as a good criterion for fossil members. The gastropod offshoot, primarily characterized by torsion, is much more difficult to define in the . fossil record, because of the great similarity of shell and muscle system between primitive representatives (Docoglossa) and the Tryblidiida. In the following discussion the origin of the Gastropoda will be reconsidered in detail. 3.2. Torsion—the key to the Gastropoda Many authors have speculated about the evolutionary process and advantages of torsion (e.g. Naef, 1913; Garstang, 1929; Eales, 1950; Morton, 1958; Fretter & Graham, 1962; GhiseIin, 1966; Thompson, 1967; Fretter, 1969; Underwood, 1972; Salvini-Plawen, 1980,1981a; Bandel, 1982; Graham, 1985; Pennington & Chia, 1985; Goodheart, 1987; Edlinger, 1988a, b). However, there is very little basic data for such a discussion. The facts known about the ontogeny and phylogeny of torsion are reviewed below. 3.2.1. Ontogeny of torsion Until this decade all theories and views on gastropod torsion were more or less based on the results of Crofts (1937,1955). The recent investigations by Bandel (1982) and Voltzow (1987) have somewhat corrected certain earlier statements (e.g. on retained larval muscles, see §2.1.) and have greatly enlarged our knowledge of this fundamental process in gastropod development.

Table 4. Definitions of the types of taxa used. Polyphyietic taxon: Taxon which does not represent a continuous lineage. This type must not be used. Monophyletic taxon: Taxon which represents a continuous lineage (respectively a continuum of generations). Holophyletic taxon (glossa* (s.m.) Haller, 1892 Superfamily *Littorinoidea* (s.m.) Gray, 1847 Superfamily Truncatelloidea Gray, 1840 (=Rissoidea Gray, 1847) Superfamily Vermetoidea (s.m.) Rafinesque, 1815 Superfamily Stromboidea (s.m.) Rafinesque, 1815 Superfamily Vanikoroidea (s.m.) Gray, 1840 Superfamily Xenophoroidea (s.m.) Troschel, 1852 Superfamily Calyptraeoidea (s.m.) Lamarck, 1809 Superfamily Lamellarioidea (s.m.) d'Orbigny, 1841 Superfamily Cypraeoidea (s.m.) Rafinesque, 1815 Superfamily Pterotracheoidea (s.m.) Ferrusac, 1821 (-Heteropoda Lamarck, 1812) Superfamily Naticoidea (s.m.) Forbes, 1838 Superfamily Tonnoidea (s.m.) Section Stenoglossa Bouvier, 1887 Superfamily Conoidea Rafinesque, 1815 (=Toxoglossa Troschel, 1847) ^ Superfamily Cancellarioidea Gray, 1853 (= Nematoglossa Olsson, 1970) Superfamily Muricoidea Rafinesque, 1815 (= Rhachiglossa Gray, 1853) Suborder CAMPANILIMORPHA nov. Superfamily Campaniloidea Douville, 1904 Suborder ECTOBRANCHIA Fischer, 1884 Superfamily Valvatoidea Gray, 1840 Suborder •ALLOGASTROPODA* Haszprunar, 1985 Superfamily Architectonicoidea Gray, 1840 ?incl. Omalogyridae G.O. Sars, 1878 (= Prionoglossa G.O. Sars, 1878) Superfamily Rissoelloidea Gray, 1850 Superfamily Glacidorboidea Ponder, 1986 Superfamily Pyramidelloidea Gray, 1840 Subclass Euthyneura Spengel, 1881 (=Pentaganglionata Haszprunar, 1985) (b) Class GASTROPODA Cuvier, 1797 Subclass Prosobranchia* Milne-Edwards, 1848 Order *Aspidobranchia* Schweigger, 1820 Suborder DOCOGLOSSA Troschel, 1866 ? Suborder 'HOT-VENT GROUP-C" Suborder COCCULINIFORMIA Haszprunar, 1987 Suborder NERIT1MORPHA Golikov & Starobogatov, 1975 7 7 Superfamily "Hot-vent group-A" (Melanodrymia) 7 ? Superfamily Neomphaloidea McLean, 1981 Suborder VET1GASTROPODA Salvinio-Plawen, 1980 Order •Pectinibranchia* Blainville, 1814 Suborder SEGUENZIINA Verrill, 1894 Suborder 'ARCHITAENIOGLOSSA* Haller, 1892 Suborder CAENOGASTROPODA Cox, 1959 Section #Cerithiimorpha* Golikov & Starobogatov, 1975 Section Ctenoglossa (s.m.) Gray, 1853 Section *Neotaenioglossa* (s.m.) Haller, 1892 Section Stenoglossa Bouvier, 1887 Suborder CAMPANILIMORPHA nov. Subclass Heterobranchia Schweigger, 1820 Cohort 'Triganglionata* Haszprunar, 1985 Suborder ECTOBRANCHIA Fischer, 1884 Suborder #ALLOGASTROPODA# Haszprunar, 1985 Cohort Euthyneura Spengel, 1881 (= Pentaganglionata Haszprunar, 1985)

429

G. HASZPRUNAR

430

(0

Class GASTROPODA Cuvier, 1797

Superorder Docoglossa Superorder "Hot-Vent Group- C" (7) Superorder N.N. ("Flexoglossata") Order COCCULINIFORMIA Order NERITIMORPHA Order 'HOT-VENT GROUP-A" (?) Order NEOMPHALIDA (?) Order VETIGASTROPODA Order SEGUENZIIDA Order N.N. ("TAENIOGLOSSA") Suborder Cyclophorina (s.m.) Suborder Viviparina {s.m.) Suborder Caenogastropoda Section •Cerithimorpha* Section Ctenoglossa (s.m.) Section 'Neotaenioglossa* (s.m.) Section Stenoglossa Suborder Campanilimorpha Suborder Ectobranchia Suborder Heterobranchia Section Architectonicoidea Section Rissoelloidea Section Glacidorboidea Section Pyramidelloidea Section Euthyneura

of gill-types and buccal system as more probable than those of the nervous system (Table 5b). In addition, the presentation of both systems (and also the cladistic and sequential ones) should demonstrate that there are always several ways to transform a phylogram into a classification so that retransformation leads exactly to the basic phylogram. According to Wiley (1979,1981) and the considerations presented above, categorical ranks are used here primarily to express hierarchy and the degree of probability of the reconstruction and not (as is more usual) to express divergence of a group. I have chosen to use 'order' for the main grades and 'suborder1 for the main offshoots. Of course one could use also 'superorder' and 'order* respectively, but I regard such questions as less important. However, it is essential to inform the reader of the classification about the differences between for example Caenogastropoda (a holophyletic

GASTROPODA (d) Docoglossa "Hot-Vent Group-C" (?) N.N. ("Rexoglossata") Cocculiniformia N.N. ("Helicoida") Neritimorpha N.N. ("Euhelicoida") "Hot-Vent Group-A" N.N. ("Skeletobranchia") Neomphaloidea Vetigastropoda N.N. ("Pectinibranchia") Seguenziina N.N. ("Taeniofllossa") Cyclophoroidea Ampullarioidea N.N. ("Planktotrophica") Caenogastropoda •Cerithiimorpha* N.N. ("Eucaenogastropoda") Ctenoglossa 'Neotaenioglossa* Stenoglossa N.N. ("Chalazaeata") Campanilimorpha Heterobranchia Ectobranchia N.N. ("Ciliotracta") Architectonicoidea N.N. ("Dextrotracta") Rissoelloidea N.N. Glacidorboidea N.N. ("Rhinophoralia") Pyramidelloidea Euthyneura

clade) and *Allogastropoda* (an orthophyletic grade) and about the consequences of this distinction. Whereas the last subgroup of a clade is most dissimilar and most distantly related to the following group (e.g. Caenogastropoda: Rhachiglossa—Campanilimorpha), the last subgroup of a grade is most similar and most closely related to the following group (e.g. 'Allogastropoda*: Pyramidelloidea—Euthyneura). Secondly, a hololphyletic group can never be ancestral to any other group, whereas an orthophyletic taxon always has ancestral status for the next taxon of equal or higher rank. Thus, the ancestor of the Euthyneura was a streptoneuran allogastropod. It is not relevant in this respect whether the respective groups are fossil or extant ones. These circumstances must be considered when reading such a clado-evolutionary system. The classification proposed here certainly is not a final one and probably never will be. New

ORIGIN AND EVOLUTION OF GASTROPODS groups of high rank will be found, new methods will add new information on the evolution of gastropods. Nevertheless it is a practical way to express the most probable phylogenetic pathway of the streptoneuran Gastropoda at the present stage of knowledge.

ACKNOWLEDGEMENTS

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