Gastropod Mollusks

(also, gastropods; class Gastropoda), a class of invertebrate animals of the mollusk type.

The body of a gastropod is divided into a head with one or two pairs of antennae and a pair of eyes, a visceral sac, and a foot. The visceral sac and the shell covering it are spirally twisted to the right or left. (A leftward twist is rarer and is found in asymmetrical animals.) Shells appear in many different shapes, ranging from high-conical to flat-spiral and patelliform. The shell consists of three layers—the horny exterior, the porcelain-like middle, and the nacreous interior. In some gastropod mollusks the shell becomes internal or disappears. The anterior portion of the visceral sac is bounded on the outside by a cutaneous fold called the mantle. The mantle cavity contains a set of primitive symmetrical organs—a rectum with an anal opening, two hypobranchial glands that perform a cleaning function, two ctenidia (gills), two osphradia (olfactory sense organs), a heart with two auricles, and two kidneys. In the process of the evolution of gastropods there occurred an increase in asymmetry, with partial or more often complete disappearance of organs, primarily in the right half of the mantle. Parallel to this there was a shift in the same organs on the left side, first forward and later partially toward the rear. In most cases the nervous system is of the dispersed-nodal type, with a high degree of ganglion concentration in the higher gastropods. There is a crossover of the visceral nerve loop (chiastoneury), caused by forward displacement of the mantle complex in the process of embryonic development. In Opisthobranchia and Pulmonata mollusks, this crossover disappears as a result of secondary unwinding (Euthyneury). The pharynx is equipped with a radula and often with jaws. In parasitic and some predatory mollusks the radula is absent. The majority of gastropod mollusks are phytophagous and saprophagous; some are also predators and a small number are parasites. They are dioecious or hermaphroditic and have one gonad each. The sex opening is on the side, close to the head. The division of ova is determined and is of the spiral type. In marine forms a trochophore-like larva, or veliger, is formed in the process of development.

Gastropod mollusks originated in the early Cambrian or Precambrian period in warmwater marine shallows with hard bottoms. They are found everywhere except in zones of solid glaciation and lowland deserts. They inhabit oceans, seas, brackish and fresh waters, and land, from high alpine meadows to great depths in the ocean, at low temperatures (in the Arctic Ocean and on the shores of Antarctica) and high temperatures. (In hot springs at temperatures up to 53° C several freshwater mollusks are found.) They occupy a large variety of ecological niches. The majority of gastropods that live in water are found near the bottom. However, there is a small number of plankton species. Some gastropods are used as commodities. (The shell or the pearl is marketed or the animal is used as food.) Some gastropods are agricultural pests (for example, some kinds of slugs). A group of them serve as index fossils. Many gastropods are the intermediate hosts of parasitic worms such as trematodes.

There are about 40,000 species in the class (about half of them extinct), which belong to three subclasses: prosobranchial mollusks, opisthobranchial mollusks, and pulmonate mollusks. In the fauna of the USSR and the bordering seas there are about 1,500 species.