What is Gastropoda?

   Gastropoda is a class of animals belonging to the phylum mollusca and represented by conches, limpets, slugs, snails, whelks, and periwinkles. Members of the gastropoda class are characterized by a shell, mantle, fleshy foot, dorsal visceral mass, and a radula (a narrow band having numerous sharp, teethlike projections on its dorsal surface which perform a rasping function). The gastropods have a distinct head, a pair of eyes, and sensory tentacles. The visceral mass, coiled into a compact structure within a shell, is asymmetrical, due to rapid growth an one side of the body and atrophy on the other. Because of coiling and atrophy the anus is anterior and located above the head. The shell of the gastropods, which is secreted by the mantle, is also coiled. The whorls may be to the right or to the left; the direction is often useful in classification. Other characters must be considered, however, because there are examples of left-coiled shells in right-coiling species. Some of the members of the group have vestigial shells and others have completely lost the shell. The young of many species pass through a trochophore larval stage (a free-swimming form that has a ring of cilia around its equator), which is also typical of some of the Annelida, Bryozoa, and other Mollusca.
Fossil evidence shows that gastropods were first present in Cambrian times. It is thought that they have reached their maximum development during the present age.

   The gastropoda class contains about 49,000 known species. which are divided into three orders: (1) Opisthobranckiata, marine animals commonly found under stones and seaweed. The body is asymmetrical, the shell and mantle vestigial or entirely missing. The animals are carnivorous; all are hermaphroditic (male and female sex organs present in one individual). Examples: sea hare, Tethys proteo; sea slug, Scyllaea pelagica. (2) Pulmonata, small freshwater and land snails that usually have a shell coiled in a simple regular spiral. Land snails close the shell during ad­verse periods (hibernation or estivation) by secreting slime containing calcium across the opening of the shell. When the slime hardens it prevenís evaporation of the body fluids; it is then known as the epiphagm. The terrestrial snails respire by means of a "lung," the inner lining of the mantle, which has a rich blood supply. Land pulmonates have two pairs of tentacles upon the head, and the eyes are located at the tips of the larger, posterior pair. Water snails have one pair of tentacles, with the eyes located at the base. Both land and aquatic forms are hermaphroditic. A few species give birth to living young but most species lay eggs. The eggs of the land dwellers are protected by hard shells, those of the water dwellers by a thick, jelly-like covering. The majority of the pulmonates are herbivorous and are excellent scavengers. Examples: river limpet, Ferrisia parallelus; slug, Limax flamis; French snail or edible snail, Helix pomatia. (3) Prosobranchiata, marine snails which have a ctenidium (a main Istem bearing feathery branches and functioning as a gill) in the mantle cavity at the front of the body. A hypobranchial gland is located in the mantle near the base of the ctenidium. In some species, e.g., Murex, the gland secretes a purple substance, the function of which is not known. The secretion of the "purple gland" was used to make Tyrian purple, a dye which was used 4,000 years ago. The dye was made by treating the juice of crushed Murex with soda and evaporating the fluid in a lead or tin receptacle. The material thus obtained was used to color the robes of kings, hence was called "royal purple."

   Nearly all of the order bear shells. The more developed members have a proboscis and a siphon which
is a tubular extensión of the mantle. The sexes are separate. Most of the species lay eggs but a few species
give birth to living young. Examples: common limpet, Acmaea testudinalis; red abalone, Haliotis rufescens; bleeding tooth, Nerita peleronta; edible periwinkle, Littorina littorea.