Nudibranchia

(invertebrate zoology) A suborder of the Opisthobranchia containing the sea slugs; these mollusks lack a shell and a mantle cavity, and the gills are variable in size and shape.

An order of the gastropod subclass Opistho-branchia containing about 2500 living species of carnivorous sea slugs. They occur in all the oceans, at all depths, but reach their greatest size and diversity in warm shallow seas.

In this, the largest order of the Opisthobranchia, there is much evidence indicating polyphyletic descent from a number of long-extinct opisthobranch stocks. In all those lines which persist to the present day, the shell and operculum have been discarded in the adult form. In many of them the body has quite independently become dorsally papillate. In at least two suborders these dorsal papillae have acquired the power to nurture nematocysts derived from their coelenterate prey so as to use them for the nudibranch's own defense. In other cases such papillae(usually called cerata) have independently become penetrated by lobules of the adult digestive gland, or they may contain virulent defensive glands or prickly bundles of dagger-like calcareous spicules. See also Opisthobranchia.