Nudibranchs, or sea slugs, belong to the phylum Mollusca, which also includes bivalves and snails. In fact, they are members of Gastropoda, the class including snails, although they have lost their shell over evolutionary time. The name nudibranch means "naked gills." In this clip you can see the gills-- they are the branched structures sticking up from the back surface of the five different nudibranchs shown, which were filmed in Lembeh Strait in Indonesia. As you can see, nudibranchs are often very brightly colored and ornately patterned.