PTEROPOD

also called sea butterfly small marine gastropods of the subclass Opisthobranchia having a pair of winglike flaps (parapodia) that are used for swimming. These gastropods were once grouped as the order Pteropoda. They live at or near the sea surface; most are less than 1 cm (0.4 inch) long. Those that lack a shell and have complicated mechanisms for capturing and eating other small animals constitute the order Gymnosomata; those having a shell and feeding by means of cilia constitute the order Thecosomata. The shell is delicate and translucent; contrasting forms are seen in the straight-needle pteropod (Creseis acicula) and the long-snouted pteropod (Cavolina longirostris).