Gastropoda

Background

The Class Gastropoda includes the snails and slugs. Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups. Gastropods are characterized by "torsion," a process that results in the rotation of the visceral mass and mantle on the foot. The result is that the mantle cavity (including anus) lies in the anterior body, over the head and mouth, and the gut and nervous system are twisted. Torsion takes place during the veliger stage, usually very rapidly. Veligers are at first bilaterally symmetric, but torsion destroys this pattern and results in an asymmetric adult. Some species reverse torsion ("detorsion"), but evidence of having passed through a twisted phase can be seen in the anatomy of these forms. Many snails have an operculum, a horny plate that seals the opening when the snail's body is drawn into the shell.

Gastropods have a muscular foot which is used for "creeping" locomotion in most species. In some, it is modified for swimming or burrowing. Most gastropods have a well-developed head that includes eyes, 1-2 pairs of tentacles, and a concentration of nervous tissue (ganglion).

Gastropods are dioecious, and some forms are hermaphroditic. Hermaphroditic forms exchange bundles of sperm to avoid self-fertilization; copulation may be complex and in some species ends with each individual sending a sperm-containing dart into the tissues of the other. Marine species have veliger larvae.

Torsion in gastropods has the unfortunate result that wastes are expelled from the gut and nephridia near the gills. A variety of morphological and physiological adaptations have arisen to separate water used for respiration from water bearing waste products.

Gastropods are by far the largest group of molluscs. Their 40,000 species comprise over 80% of living molluscs. Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior. Some graze, some browse, some feed on plankton, some are scavengers or detritivores, some are active carnivores.