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Gastropoda

The largest and most varied class in the phylum Mollusca, possibly numbering over 74,000 species and commonly known as snails.  See also: Mollusca

Functional Morphology

The shell is in one piece that, in the majority of forms, grows along a turbinate (equiangular) spiral (Fig. 1) but is modified into an open cone in various limpets or is secondarily lost in various slugs.

Fig. 1  Typical turbinate shell of gastropods. (a) Longitudinal section ground through the shell of a specimen of Conus purius to reveal the central columella and spiral of whorls expanding to the aperture. (b) Diagram of the shell to show the relationship of the whorls sectioned. (c) Stylized representation of the conchospiral line, as viewed with the shell apex tilted toward the observer. This logarithmic or equiangular line is the center line of a steadily expanding conic tube secreted by the mantle edge (at the shell aperture) during growth. (After W. D. Russell-Hunter, A Life of Invertebrates, Macmillan, 1979)

Effects of torsion

 All gastropods, at some time in their phylogeny and at some stage in their development, have undergone torsion. The process does not occur in any other mollusks. It implies that the visceral mass and the mantle shell covering it have become twisted through 180° in relation to the head and foot. As a result of torsion, all internal organs are twisted into a loop. Similarly in gastropods, the mantle cavity (the semi-internal space enclosed by the pallium or mantle) containing the characteristic molluscan gills (ctenidia) has become anterior and placed immediately above and behind the head. The most primitive gastropods, like stem stocks in the other molluscan groups, retain a pair of aspidobranch (bipectinate or featherlike) gills, each with alternating ctenidial leaflets on either side of a ctenidial axis in which run afferent and efferent blood vessels (Fig. 2). Lateral cilia on the faces of the leaflets create a respiratory water current (toward the midline and anteriorly) in the direction opposite to the flow of blood through the gills, to create the physiological efficiency of a countercurrent exchange system.  See also: Countercurrent exchange (biology)

As also occurs in other mollusks, the ctenidia form a gill curtain which functionally divides the mantle cavity into an inhalant part (lateral and below) with characteristic osphradia as water-testing sense organs, and an exhalant part into which the anus and renogenital ducts discharge. In zygobranch (two-gilled) gastropods, various shell slits or complex openings have been developed (Fig. 2c and d) to deal with the problems of sanitation created by the mantle cavity being brought anteriorly above the head. In various ways, these accommodate the exhalant stream of deoxygenated water and the feces and genital and kidney products which accompany it, so that this exhalant discharge is not directly over the head. In the majority of gastropods this problem is resolved by the reduction of the gill pair to a single ctenidium (Fig. 3) or to a pectinibranch ctenidium (a one-sided, comb-shaped “half-gill”).

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Fig. 3  Further evolution of the mantle cavity in gastropods. The mantle cavity and pallial complex is shown (a) in dorsal view and (b) in cross section for an asymmetric snail with a single aspidobranch gill, and (c, d) for an asymmetric snail with a “half-gill” or pectinibranch ctenidium. (e) The mantle cavity of a pulmonate land snail in dorsal view shows that there is now no gill and the mantle has become vascularized as a lung wall. (After W. D. Russell-Hunter, A Life of Invertebrates, Macmillan, 1979)

Shell coiling and asymmetry

 In a typical snail the shell is turbinate or helicoid, growing as a steadily expanding conic tube along the “screw spiral” of a logarithmic or equiangular line on the surface of a hypothetical cone (Fig. 1). Only a minority of gastropods have planospiral shells or the open limpet form. The majority with turbinate shells cannot have similar left and right halves, and almost all snails are anatomically asymmetric. Even in those gastropods in which the shell is lost and there has been a secondary return to a bilateral symmetry of external features, there remain marked asymmetries of internal anatomy. The majority of turbinate-shelled gastropods have dextral coiling, with a consequent reduction of the pallial organs of the right side, so that the left ctenidium persists. Internal organs are similarly reduced, so that “higher” gastropods have only the left auricle of the heart (the monotocardiac condition), and the left kidney and a single gonad opening to the exterior by the originally right-hand renogenital coelomoduct.

Mantle cavity evolution

 From the most primitive forms which retain most clearly the basic effects of torsion (Fig. 2), the further evolution of the gastropods has involved increasing asymmetry of the mantle cavity organs. Relatively few living gastropods are zygobranch with two ctenidia (and the diotocardiac condition, Fig. 2a). These include keyhole limpets such as Fissurella and abalones (Haliotis).

In most gastropods the right ctenidium is completely lost, and the pallial water flow has become inhalant from the left side of the snail, with the anus and renogenital openings moved over to the right (now exhalant) side (Fig. 3a and b). A minority of these single-gilled snails retain an aspidobranch gill, with some danger of particulate material clogging the dorsal part of the mantle cavity. Such snails include the limpet genus, Tectura (=Acmaea), common on both Atlantic and Pacific coasts, the big worldwide group of top shells (Trochus and its allies), and the tropical littoral genus Nerita. All these forms are ecologically limited to relatively clean water over hard substrata and are unable to invade areas of the sea bottom or seashore covered with mud or silt.

By far the most successful marine gastropods (without such ecological limitation) are those in which the pallial structures are further reduced (Fig. 3c and d), with a pectinibranch ctenidium whose axis is fused to the mantle wall, resulting in greater hydrodynamic efficiency. Note that this “half-gill” pattern still presents the same functional relationships of ciliary water currents and blood vessels in a countercurrent system. Most familiar snails of the seashore, including such genera as Busycon (whelks), Nassarius (mud snails), Littorina (periwinkles), Euspira (=Polinices) [moon snails], and very many others, have pectinibranch gills and this highly asymmetric arrangement of the mantle cavity.

A final reduction of pallial structures is found in the pulmonate snails and slugs (Fig. 3e), in which the mantle cavity is an air-breathing lung and there are no ctenidia. The other more specialized subclass, the Opisthobranchia, shows detorsion after loss of the shell and a variety of secondary (neomorphic) gills, especially in the extremely beautiful sea slugs or nudibranchs.  See also: Nudibranchia; Pulmonata

 Diversity and Classification

 More than half of all molluscan species are gastropods, and they encompass a range from the marine limpets, which can be numbered among the most primitive of all living mollusks, to the highly evolved terrestrial air-breathing slugs and snails. Pulmonates and certain mesogastropod families are the only successful molluscan colonizers of land and freshwaters. The anterior mantle cavity (resulting from torsion), as opposed to the posterior mantle cavity of all other major molluscan stocks, remains diagnostic of the class despite a diversity of functional morphology unequaled by any comparable group in the entire animal kingdom.

Presently classification of gastropods is rather unsettled, even though the phylogeny of this group is being examined vigorously. Consequently, an older arrangement of gastropod taxa, in practice prior to the onset of phylogenetic systematics, is presented here. While most of these taxa are now considered to be artificial, they are useful to consider in that each one demonstrates parallel morphologies and habitats.

The older systematic arrangement of the class Gastropoda involves three somewhat unequal subclasses: Prosobranchia, Opisthobranchia, and Pulmonata.

Prosobranchia

The largest and most diverse subclass is Prosobranchia, which is made up largely of marine snails, all retaining internal evidence of torsion. The prosobranchs have traditionally been divided into three orders: Archaeogastropoda, Mesogastropoda, and Neogastropoda. The primitive Archaeogastropoda are characterized by the presence of one or two aspidobranch (bipectinate) ctenidia, auricles, metanephridia (kidneys), and osphradia (Figs. 2, 3a and b). Members of the polyphyletic Mesogastropoda, which comprises almost 100 families, have only one pectinobranch (monopectinate) ctenidium (left), auricle, and nephridium (Fig. 3c and d). Members of the Neogastropoda, which appears to be monophyletic, also exhibit these characters, but their shell has a canal or notch that holds a tubular extension of the mantle (siphon).

In some current classifications, most families of the Mesogastropoda and Neogastropoda (and some Archaeogastropoda) constitute a taxon known as the Caenogastropoda, whereas most of the archaeogastropods are placed within the Patellogastropoda (true limpets) and Vetigastropoda (abalones, top and turban snails, keyhole limpets).  See also: Mesogastropoda; Neogastropoda; Prosobranchia

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Opisthobranchia and Pulmonata

 The other two subclasses (Opisthobranchia and Pulmonata) are each considerably more uniform than the subclass Prosobranchia and, in both, the effects of torsion are reduced or obscured by secondary processes of development and growth. Even so, in newer classifications they are often combined into a single group (Heterobranchia) along with a few groups (sundials, pyramidellids) formerly classified as prosobranchs.

The marine subclass Opisthobranchia consists largely of sea slugs, in which the shell and mantle cavity are reduced or lost and there is a bilaterally symmetrical adult with a variety of secondary gill conditions.  See also: Opisthobranchia

The final subclass, Pulmonata, consists of gastropods with the mantle cavity modified into an air-breathing lung and with no ctenidia (Fig. 3e). There are a few littoral marine forms, but the order Basommatophora is mainly made up of the freshwater lung-snails, and the order Stylommatophora consists of the successful land snails such as Helix plus a few shell-less families of land slugs.  See also: Pulmonata; Basommatophora; Stylommatophora

Bibliography

 V. Fretter and A. Graham, British Prosobranch Molluscs, 2d ed., 1994

F. W. Harrison and A. J. Kohn (eds.), Microscopic Anatomy of Invertebrates, vol. 5, 1994, vol. 6B, 1997

R. N. Hughes, A Functional Biology of Marine Gastropods, 1986

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R. M. Linsley, Shell form and the evolution of gastropods, Amer. Sci., 66:432–441, 1978

W. D. Russell-Hunter, A Life of Invertebrates, 1979

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J. Taylor (ed.), Origin and Evolutionary Radiation of the Mollusca, 1996

E. R. Trueman and M. R. Clarke, The Mollusca, vol. 10, 1985

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Additional Readings

 R. Bieler, Gastropod phylogeny and systematics, Annu. Rev. Ecol. Syst., 23:318–338, 1992

J. T. Pennington and F. Chia, Gastropod torsion: A test of Garstang's hypothesis, Biol. Bull., 169:391–396, 1985

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W. F. Ponder and D. R. Lindberg, Towards a phylogeny of gastropod mollusks: An analysis using morphological characters, Zool. J. Linn. Soc., 119:83–265, 1997

S. M. Stanley, Gastropod torsion: Predation and the opercular imperative, Neues Jahrb. Geol. Palaeontol. Abh., 164:95–107, 1982

 Ali Fazeli = egeology.blogfa.com

E. E. Strong, Refining molluscan characters: Morphology, character coding and a phylogeny of the Caenogastropoda, Zool. J. Linn. Soc., 137:447–554, 2003

P. J. Wagner, Gastropod phylogenetics: Progress, problems, and implications, J. Paleontol., 75:1128–1140, 2001

Gastropod Classification

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Tree of Life: Gastropoda

Marine Gastropods

Freshwater Molluscan Snails

Animal Diversity Web

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 Ali Fazeli = egeology.blogfa.com