Nudibranchs, also known as sea slugs, are soft-bodied marine snails belonging to the suborder Nudibranchia, the largest suborder of the order Opisthobranchia. There are more than 3,000 described species.
The adult form is without a shell or operculum (a bony plate covering the opening of the shell, when the body is withdrawn).
The word "nudibranch" comes from Latin nudus meaning "naked", and Greek brankhia meaning "gills". The name is appropriate since the dorids (infraclass Anthobranchia)
breathe through a branchial plume of bushy extremities on their back,
rather than using gills. By contrast, on the back of the aeolids in
infraclass Cladobranchia there are brightly colored sets of tentacles called cerata.
Nudibranchs have cephalic (head) tentacles, which are sensitive to touch, taste, and smell. Club-shaped rhinophores detect the odors.
They are hermaphroditic, but can rarely fertilize themselves.
Nudibranchs typically deposit their eggs within a gelatinous spiral.
They are carnivorous. Some feed on sponges, others on hydroids, others on bryozoans, and some are cannibals, eating other sea slugs, or, on some occasions, members of their own species. There is also a group that feeds on tunicates and barnacles.
Body forms can vary wildly. They lack a mantle cavity. Their size varies from 40 to 600 mm.
They occur worldwide at all depths, but they reach their greatest size and variation in warm, shallow waters.
Among them can be found the most colorful creatures on earth.
Because sea slugs, in the course of evolution, have lost their shell,
they have had to evolve another means of defense: camouflage, through color patterns that make them invisible (cryptic behavior) or warn off predators as being distasteful or poisonous (aposematic behavior). Champions in their colorful display are the Chromodorids. The nudibranchs that feed on hydroids store the hydroid's nematocysts (stinging cells) in the dorsal body wall. This enables the nudibranch to ward off potential predators.
Information from Wikepedia - http://en.wikipedia.org/wiki/Nudibranch