Archerd Shell Collection > Shell Classes > Gastropoda Family Index
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OverviewThe Gastropod Class. This class includes most molluscs, with about 30,000 existing species and 15,000 fossil forms. Generally, the gastropods have an asymmetrically spiral shell, which functions as a portable retreat. Their body consists of a head, foot, visceral hump, and mantle. Visceral organs show well organized features, which include a nervous system, a heart & circulatory system, a digestive & excretory system, and a reproductive system (for details on the latter, see Morton, 1979). Both sexual and hermaphroditic reproduction are found among the various species and families. At the head end, the mouth is surrounded by one or two pairs of tentacles which often carry eyes either at the base or on stalks while the pharynx usually bears coarse or fine teeth on the tongue (radula), depending on its mode of living. The foot is usually in the form of a flat, creeping sole, which may bear an operculum for closing the shell. The shell, itself, is to some extent contoured to the shape of body appendages. Nevertheless, extensive variations are found in the gastropod's basic body plan, chief among them being the loss of bilateral symmetry in the visceral organs. The classification system attempts to group animals according to the extent of such modifications and/or specialization from presumably simpler ancestral forms, as noted in the three subclassses below. The Prosobranch Subclass is by far the largest with over 30,000 species. All have a big anterior mantle cavity into which the mollusc can withdraw, closing it off with an operculum. There are separate sexes and most live in marine waters. The prosobranchs are extremely diverse and, the several taxonomic orders reflect increasing degrees of specialization. The order, Archeogastropoda, is evolutionarily the simplest, with molluscs showing 2 gills and usually a double chambered heart. They are exclusively grazers of fine particles and/or herbivores. Gametes are discharged directly to the water. The orders, Caenogastropoda and Heterogastropoda, show far greater specialization, and most are adapted to an aggressively carnivorous life style. All have a single chambered heart, one gill, a penis for internal fertilization, more complex nerve ganglia, and well structured sensory organs. The Opisthobranch Subclass includes snails that have either no shell, or a shell too small to completely enclose the mollusc. There is usually no operculum and the animals are hemaphroditic. Only 4,000 opisthobranch species are known, and they include the highly colorful sea slugs (nudibranchs). They are considered more evolutionarily advanced, carnivorous, and equipped with a complex nervous system that involves consolidation and development of most ganglia in the head region. In the higher orders of these opisthobranchs, as well as pulmonates discussed below, modern research has demonstrated a capability for learned behavior with respect to feeding responses as well as memory retention of up to 4 months (Beesley et al., 2000). The opisthobranchs also show partial to nearly complete reversal of the torsion that is so conspicuous in the prosobranch subclass. The Pulmonate Subclass includes mainly freshwater and land snails, but a few species are marine. Pulmonates number about 28,000 species. Uniquely, this subclass features a vascularized mantle cavity, which serves as a lung sac for air breathing. They have no gills, and they reproduce hermaphroditically. For pictures, see Land Snail family page. |
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Archerd Shell Collection > Shell Classes > Gastropod Family Index |
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