The phylum Mollusca consists of diverse group of organisms which include slow-moving snails and slug, bivalve clams, and highly active cephalopods. The phylum includes over 50,000 living species and is the second largest phylum of invertebrates. Giant squid is the largest invertebrate animal.
Mollusks also show a great diversity of form but all are built on the same basic plan.
Mollusks are triploblastic coelomates which exhibit bilateral symmetry. Most animals possess shell.
The body is covered by a glandular epithelial envelope called mantle which secretes calcareous shell. The shell is protective, however it is handicap to locomotion, therefore some of the more active mollusks show a reduction or loss of shell.
Mollusks are widely distributed. Some groups are exclusively aquatic (e.g., cephalopoda), freshwater or marine. The others include terrestrial animals (land snail) living mostly in moist places.
The body is unsegmented and soft. The body can be divided into head, a ventral muscular foot and a dorsal visceral mass containing most of the internal organs. Over the visceral mass mantle is present which secretes a shell. The space between the shell and mantle cavity contains gills in some animals. In the mouth cavity of many mollusks there is a rasping tongue-like radula provided with many horny teeth.
The body is highly organized with complex digestive, respiratory, circulatory, excretory, nervous and reproductive systems.
Digestive system consists of gut with two openings, the mouth and the anus.
The excretory organs are paired nephrida.
Except for Cephalopoda, the circulatory system is open. The coelom is divided into sinuses or blood spaces. Heart pumps the blood into the sinuses. A respiratory pigment of blue in color, called haemocyanin is present.
The gaseous exchange is by gills mostly. In some cases such as snail, the mantle cavty is converted into a lung.
The nervous system consists of three pairs of interconnected ganglia present in the head, foot and body regions.
The organ of locomotion is a muscular foot; however in many species the movement is slow. The other are sessile i.e. unable to move.
The sexes are separate. Trochophore larva develops during embryological development.