Aplysia belongs to Phylum Mollusca which is the second largest animal phylum. The organization seen in molluscs is of organ system level and they can be terrestrial or aquatic. Features of molluscs are that they are bilaterally symmetrical, triploblastic and coelomate animals. In molluscs the body is covered by calcareous shell and is unsegmented with distinct head, visceral hump and muscular foot. The mantle over the visceral hump is formed by soft and spongy layer of skin. There are feather like gills present in the mantle cavity (space between the hump and the mantle). Examples are Cuttle fish, Squid, Octopus, etc.