The Class: Gastropoda
includes most molluscs, with perhaps 60,000 existing species and 15,000 fossil forms. Generally, the gastropods have an asymmetrically
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
heart & circulatory system,
a digestive & excretory system, and a reproductive system (for details on the latter, see
). 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 (
, 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.
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 the Group and two Subclasses below.
The Subclass: Prosobranchia
includes most gastropods except the Group: Opisthobranchia, and the Subclass: Pulmonata
. Thr prosobranchs are 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 clades reflect increasing degrees of specialization.
The earliest evolutionary forms (formerly the Order:Archeogastropoda) show double gills, double visceral organs, and usually a double-chambered heart. Reflecting these changes, current classification has replaced the former Order: Archaeogastroda with several newly named Clades, specifically Patellogastropoda, Vetigastropoda, and Cycloneritimorpha (see
pages). There is a progessive trend among the superfamilies of these clades toward single-sided visceral organs. All species within these groups are grazers of fine particles and/or herbivores. Gametes are discharged directly to the water, and specialized sexual organs are absent.
A Clade is defined as a group of species derived from a common ancestor, based not only on shell and soft tissue anatomy, but also on DNA, RNA, and certain other shared features. Species within the higher Clades, e.g., Littorinimorpha and Neogastropoda (formerly 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-like organ for internal fertilization, more complex nerve ganglia, and well structured sensory organs.
Unlike a Clade, an Informal Group may include certain species from more than one clade, but where the member species have common shell or anatomic features.
The Informal Group: Opisthobranchia
(formerly a 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, based on carnivorous feeding, a more complex nervous system, and the consolidation and further development of major ganglia in the head region.
Certain species among the several clades comprising this informal group, and the pulmonates
have been shown to possess 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 Subclass: Pulmonata
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 further details and pictures, see