Phylum Mollusca is a very successful and diverse group of animals. Considered to be the second largest group of animals with regard to species number. These are soft bodied animals without segmentation. The body is divided into head, muscular foot and visceral mass. The body is covered by a mantle and a shell.
Respiration happens through gills (ctinidia) in the mantle cavity. The most common larva is a trochophore larva.
There are seven classes of which three are more prominent.
Class Pelecypoda or Bivalvia:– These are aquatic molluscs having bivalves. They burrow in mud and sand. The body is laterally compressed. (eg) mussels, clams, oysters.
Class Gastropoda:- These are either aquatic or terrestrial molluscs. They posses a spiral shell. The foot is large and flat. They have well developed head with tentacles and eyes. (eg) snails, slugs, and limpets.
Class Cephalopoda:- These are mostly marine. They are adapted for swimming. The foot is modified into eight to ten long tentacles in the head region. The shell is either internal or absent. (eg) Octopus, Loligo, Sepia.