Animals small to extremely large (Aplysia vaccaria can reach about 1m length) with, in some genera, an atrophied inner shell. The species delineation is almost impossible with this shell only.
 

Shell small, flat. Protoconch bulbiform; teleoconch of about one whorl, extremely enlarged, of a horny appearance; margins thin and fragile.
 

Shell small, domed. Protoconch bulbiform; teleoconch of about one whorl, rather enlarged, of a horny appearance; margins thin and fragile.