The gastropods, AKA gasteropods or univalves, are a class of mollusks, composed mostly of snails and slugs, but also including limpets, the sea snail, abalone, cowries, conches, periwinkles, whelks, sea slugs, etc. Most live in marine waters, but members of this class can be found everywhere. Gastropod is the second largest class of Animals after Insecta.
The primary characteristic of gastropods is torsion of the digestive system; in which the visceral mass twists around so that the anus comes out above the head.
Gastropods are split into two subclasses, Eogastropoda (the true limpets) and the much, much larger group Orthogastropoda, which contains everything else.