Meaning "naked gills," the name Nudibranchia refers to externally exposed respiratory organs found in the form of a tuft-like branchial plume on the back end of a slug or to a series of club-shaped projections along its dorsum. True sea slugs, they are set aside from the other orders (except Sacoglossa) by the absence of a shell in the adult form. This order is by far the largest of the subclass Opisthobranchia in the world.