Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups. 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. Many snails have an operculum, a horny plate that seals the opening when the snail's body is drawn into the shell. In some, it is modified for swimming or burrowing.
Gastropods are dioecious, and some forms are hermaphroditic.
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. Gastropod feeding habits are extremely varied, although most species make use of a radula in some aspect of their feeding behavior.