Mollusca is the second largest animal phylum. Molluscs are terrestrial or aquatic (marine or fresh water) with an organ-system level of organisation. They are bilaterally symmetrical, triploblastic and coelomate animals. The body is covered by a calcareous shell and is unsegmented, with a distinct head, muscular foot and visceral hump. A soft and spongy layer of skin forms a mantle over the visceral hump. The space between the hump and the mantle is called the mantle cavity, in which feather-like gills are present; they have respiratory and excretory functions. The anterior head region has sensory tentacles. The mouth contains a file-like rasping organ for feeding, called the radula. Molluscs are usually dioecious and oviparous, with indirect development. Examples include Pila (apple snail), Pinctada (pearl oyster), Sepia (cuttlefish), Loligo (squid), Octopus (devil fish), Aplysia (sea-hare), Dentalium (tusk shell) and Chaetopleura (chiton). The key identification features are the calcareous shell, mantle and mantle cavity, the radula and the unsegmented body divided into head, foot and visceral hump. These points regularly appear in NEET as match-the-column items, and the fact that molluscs are second only to arthropods in number is also commonly tested.