• Body type : Body is soft , unsegmented , Bilate rally symmetrical. • Body parts : Body consist of head , foot, mantle and visceral mass. • Shell : Body is protected by exoskeleton. calcareous shell secreted by mantle. • Head : Head is distinct , bearing the mouth and provided with eyes , tentacles and other sense organs. • Respiratory organs : Respiratory organs consist of numerous gill or ctenidia usually provided with osphradium at the base. Lung is developed in terrestrial forms. • Body cavity : Body cavity is haemocoel. • Radula : pharynx contains a raspindg organ, the Radula . • Excretory organs : Excretory organs consists of pair of metanephridia. • Sex : Sexes usually separate (dioecious) but some are hermaphroditic. • Fertilisation : Fertilisation is external or internal. • Development : Development is either direct or with metamorphosis through trochophore stage.