GASTROPOD

any member of the class Gastropoda, the largest group of the phylum Mollusca, consisting of about 65,000 species. Gastropod, which means belly-footed, refers to the broad tapered foot on which these animals glide. The class comprises the snails, which have a shell into which the animal can withdraw, and the slugssnails whose shells have been reduced to an internal fragment or completely lost in the course of evolution. Gastropods are among the few groups of animals to have successfully radiated in the ocean, fresh waters, and on land. Because of the challenges presented by these diverse habitats, gastropods are very difficult to characterize. A few are used as food, a very few transmit animal diseases (only a fraction of these have been found to carry the agents of human and animal disease), and the shells of some are used as ornaments or in making jewelry. The main role of gastropods is as scavengers, feeding on dead plant or animal matter, or as predators. any member of the class Gastropoda, the largest group of the phylum Mollusca, consisting of about 60,000 species. Gastropods are distributed worldwide, occurring in most marine, freshwater, and terrestrial environments. A typical gastropod, such as a sea snail, has a large foot with a flat sole for crawling, a single coiled shell that covers the viscera, and a head bearing a pair of eyes and tentacles. During development, the viscera, in many species rotate 180 relative to the head and foot (in a process called torsion) so that the gills and anus lie above the head. Gastropods are so diverse that there are many exceptions to these group characteristics. Sea slugs and garden slugs, for example, lack shells, while the species of one genus of snails have shells consisting of two halves like that of a bivalve (q.v.). Most gastropods feed by using a radula, a ribbon of small horny teeth that tear the food into pieces that are then collected by the lips. In many marine snails, the mouth and radula are situated at the end of an extensible proboscis (i.e., a tubular sucking organ). Gastropods may be herbivorous or carnivorous, predatory or parasitic, or they may feed on plankton and deposits of detritus filtered from the water. Gastropods are commonly divided into three major groups: prosobranchs, opisthobranchs, and pulmonates. The prosobranchs include most shelled marine snails as well as some terrestrial forms; they characteristically have gills in the cavity of the body wall (mantle) and a lidlike structure (operculum) attached to the foot for closing the opening of the shell. The opisthobranchs, which include seahares, nudibranchs (seaslugs), and bubble shells, either lack a shell or have a reduced shell that is partly embedded in the body. The pulmonates, consisting of most land and freshwater snails and slugs, have no gills and use the mantle cavity as a lung. For further information on members of the gastropod group, see opisthobranch; prosobranch; pulmonate. Additional reading Vera Fretter and Alastair Graham, British Prosobranch Molluscs: Their Functional Anatomy and Ecology (1962), provides essential reference material on the gastropod class. Libbie Henrietta Hyman, The Invertebrates, vol. 6, Mollusca I (1967), contains a classic summary of gastropods. An introduction to land snail systematics and biology is found in M.P. Kerney and R.A.D. Cameron, A Field Guide to the Land Snails of Britain and North-West Europe (1979). W.F. Ponder (ed.), Prosobranch Phylogeny (1988), is a summary by a number of researchers. Vera Fretter and J. Peake (eds.), Pulmonates, 2 vol. in 3 (197579), examines this subclass in detailsee especially the essay by A. Solem, Classification of the Land Mollusca, in vol. 2A, pp. 4997, which provides an outline of taxa. Another source is Roger N. Hughes, A Functional Biology of Marine Gastropods (1986). Evolution and paleontology The basic trends in snail evolution involve changing from a herbivorous to a carnivorous diet, shifting from the ocean to freshwater and terrestrial life, and adopting a sluglike form through reduction or loss of the shell and visceral hump. Each change has occurred independently several times in the course of gastropod evolution. Prosobranch gastropods are the most primitive. One group, the Diotocardia, which retains two sets of mantle organs, is nearest the generalized gastropod in structure. Gradual loss of the right set of mantle organs occurs in the primitive archaeogastropod superfamilies Trochacea and Neritacea, thus providing a transition to the more highly developed order Monotocardia, with only one set of mantle organs. Among the numerous changes in the Monotocardia are fewer radular teeth and a shift from grazing on algae and fungi to predation and the consumption of larger sessile organisms. The two main divisions of the Monotocardia show different evolutionary patterns. Although most mesogastropods have remained coastal marine, several families moved into fresh water. Others crossed to land directly from the tidal zone, rather than passing through a freshwater transitional period. At the peak of prosobranch evolution is the predatory suborder Stenoglossa, all marine inhabitants, with highly modified radular teeth and often well-developed poison glands to aid in capturing prey. Reduction and loss of the right mantle organs are correlated with more efficient respiration and sensory apparatuses, in which a water current crosses over the sensory organs and gills on the left side, then out on the right side, together with excretory and fecal deposits. Opisthobranchs probably arose from an unknown group of primitive prosobranchs and have evolved extensively into different lines showing a reduction of the visceral hump and shell. In certain forms the foot is shortened, and external cerata develop to provide a respiratory surface to replace the lost mantle-cavity surface. Such opisthobranch groups as the order Pyramidellacea contain a mixture of prosobranch and opisthobranch characteristics. Pulmonates show varying degrees of adjustment to freshwater and land life, with increasing union of the male and female gonoducts characterizing the more advanced groups. Similarly, the highly advanced suborder Holopoda and superfamily Limacacea show complex accessory organs on the genitalia and a more sophisticated means of water conservation through development of a closed secondary ureter and resorption of water from the excretory products. More than a dozen different groups of pulmonates have become predators, usually upon other snails or earthworms. Fossil gastropods are known from Cambrian deposits. Since the shell is often very similar in unrelated families, fossil gastropods more than 350 million years old are not usually placed in the classification outlined below but instead are treated separately. Most stenoglossan prosobranchs appeared near the end of the Mesozoic (66.4 million years ago), and many groups of land snails are known from Eocene formations (57.8 to 36.6 million years old). Snails had their adaptive radiation early in geologic history. Living genera of marine, freshwater, and land snail families are known from Oligocene to Miocene deposits (36.6 to 5.3 million years old). Unlike mammals, who have undergone great evolutionary change in the last 50 million years, gastropods have shown little progressive evolution during that time. Classification Since the 1980s, gastropod classification has been the subject of extensive debate. Major revisions based on detailed information about traditional anatomy and shell features have been challenged by cladistic attempts to identify changes that have taken place once in the evolutionary history of a group and thus derive phylogenetic schemes, attempts to delineate the genealogy of groups based primarily on neurological structures. Both traditional and cladistic classification schemes are being tested by data from molecular studies. Given the antiquity of the gastropods as a group, however, it is perhaps realistic to expect that most changes have occurred more than once. Graham gave an excellent review of anatomic and functional trends, concluding that many of the groups historically recognized as advanced are grades reached by several taxa independently, not monophyletic clades (groups with the same ancestor). A conservative classification is presented below, basically using concepts from Vera Fretter and Alistair Graham (1962) for the prosobranchs, Louise Schmekel (1985) for the opisthobranchs, and Alan Solem (1978) for the pulmonates, with minor modifications from a number of sources. The remarkable radiation of primitive limpets associated with the hydrothermal vents was still under analysis in the late 1980s and is not included. The pattern of change in the respiratory and nervous systems and in the mantle complex is fundamental. Among the major trends are the change from a streptoneurus (twisted) to a euthyneurus (straight) nerve loop, the progressive loss of the right part of the primitively paired mantle organs, the increasing sophistication of hermaphroditism, the tendency toward the reduction of the visceral hump and shell, the multiple changes from a herbivorous to a carnivorous diet, the shifts from marine to freshwater or land environments and occasionally the return from the land to the ocean, and the shifts in locomotor mechanics.