The nudibranch, S. pelagica, is often called a sea slug. The shell and the mantle cavity have been completely lost, and only secondary gills are present (Mill 1972). They are bilaterally symmetrical and have two pairs of sensory organs (tentacles) near the head, an anterior pair of cephalic tentacles, and a posterior ring of tentacles. These tentacles don't aid in capturing prey, but are sensory organs or aid in respiration. They resemble leaf-like lobes, a form of cryptology in their environment. S. pelagica is orange-brown and yellow in color, and is about 3 to 4 inches (7-10 cm) in length (Fatheringham and Brunemeister 1989).