Illustrated here are two nudibranchs, superclass Opisthobranchia. These particular individuals were photographed off Lizard Island on the Great Barrier Reef, Australia.

These animals belong to the phylum Mollusca, which, after the Arthropoda, contains the greatest number of described living species (100,000), plus an additional 60,000 known fossil species. About half of the species are marine, the rest being freshwater or terrestrial. The molluscs are extremely diverse in form including groups such as the polyplacophorans (chitons), gastropods (snails, nudibranchs, sea hares etc), bivalves (clams, oysters etc) and the cephalopods (squids, octopods & the chambered Nautilus). Despite such diversity all molluscs are derived from the same fundamental body plan which is typically divided into a head, with well developed sensory organs, a large, soft visceral mass, from which the phylum gets its name, and a muscular foot.

At first glance, the Opisthobranchs, with some 2,000 species worldwide, hardly seem to be gastropods. Their often large bodies do not have sizable external shells, but they do exhibit the basic gastropod plan. The Nudibranchia or sea slugs are perhaps the most conspicuous molluscs. They are generally brightly coloured, and can range in size from less than one eigth of an inch to over one foot in legnth. They have no shell and the body is bilaterally symmetrical. The head always has a pair on antennae-like rhinopores. Most have gills on the posterior part of the body and some can retract their gills into a branchial pocket. The upper surface of nudibranchs often have cerata, digitate or club-like projections of tissue, that can be brightly coloured. These are used in respiration, defense and digestion.