Also found in: Dictionary, Thesaurus, Medical, Wikipedia.
a subclass of gastropod mollusks. The most primitive opisthobranches have an external shell and a muscular foot with lateral processes (parapodia). In the majority of opisthobranches the shell is overgrown by the mantle and is partially or completely reduced. The internal sac is also reduced. Complete secondary bilateral symmetry is characteristic of many opisthobranches. They are hermaphrodites. The majority of species have a swimming larva called a veliger. There are 11 orders, which include about 10,000 species. They inhabit all depths of the ocean; only a few live in fresh waters. The majority are benthonic. There are predacious and herbivorous forms.