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 Most gastropods have a single, usually spirally coiled shell into which the body can be withdrawn, but the shell is lost or reduced some important groups.
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.