Jain Biology: 8. Phylum Mollusca:

They usually possess a shell of calcium carbonate; unsegmented body is divisible into head, foot and visceral mass. The digestive tract is complete; the circulatory system is almost always well developed and a dorsal heart; respiratory organs are galls or ctenidia or it is by mantle; excretion is by kidneys; nervous system consists of the cerebral pair in the head, pedal pair in the foot, and visceral in the body region. Sexes are usually separate but some are bisexual. Examples of Mollusca—snail, mussel, oyster, cuttle fish and octopus.