any of various marine snails of the order Cephalaspidea (subclass Opisthobranchia of the class Gastropoda). These snails characteristically have thin, globular shells; in some species the shells are embedded in the animal's body.

Many of these snails are active predators, feeding on other gastropods as well as on bivalves and polychaete worms, which they swallow whole and then crush between gizzard plates of calcareous material. Some of the larger forms, such as the genera Bulla, Haminoea, and Akera, are, however, herbivorous, feeding largely on green algae. Bubble shell snails are simultaneous hermaphrodites (i.e., functional reproductive organs of both sexes occur in the same individual); most deposit their fertilized eggs in jellylike strings on mud or seagrasses. Bubble shells are found in all the oceans. Species of Bulla are particularly common in shallow-water seagrass meadows of the tropics.