Molluse= soft They have soft, unsegmented and muscular body. They are aquatic (usually marine, some are fresh water) and some are terrestrial found in damp and moist places. They are triploblastic i.e. body contains 3 germs layers- ectoderm, mesoderm and endoderm. They are bilaterally symmetrical except gastropods (E.g. Helin). Gastropods are asymmetrical due to torsion (i.e. coiling of internal organs). They are coelomate and body contains haemocoelom. They possess organ-system grade of body organization. Body is covered by calcareous shell and divided into head, mantle, foot and visceral mass. They exhibit holozoic form of nutrition. They have a complete and straight alimentary canal. Digestion is extra-cellular. Respiration occurs by gills or ctendia (inaquatic forms) and lungs in terrestrial forms. Excretion occurs by metarephridilla or kidney. Circulation is of opened type i.e. blood flows through open spaces inside body not in blood vessels. Locomotion takes place by the muscular foot`s contraction and relaxation. Sensory organs are eye, tentacles, osphradium, etc Development is indirect i.e. larval form is known as trocophore larva or Veligar larva.
On the basis of nature and type of muscular foot, they are classified as:- Monoplacophora: They are marine forms. Head bars tentacles. Body is symmetrical with dome shaped mantle. Foot s disc like with a flat, creeping sole. The shell is flattened limpet-shaped with spirally coiled protoconch. Example: Neopilina Amphineura: They form dorso-ventrally flattened body. A small head is present. It is not distinct from the rest of the body. Eyes and tentacles are absent. Shell may be absent or consists of a row of plates. Foot may be large, reduced or absent. Example: Chitan Scaphodia (boat-like &pointed and used for burrowing feet) They are exclusively marine. Body is elongated and enclosed in the tusk like shell. The shell is tubular and opens at both ends. Head is rudimentary. Eyes are absent. Foot is elongated and conical. It is used for digging. Example: Dentalium. Gastropoda (ventral muscular foot rising from a stomach) They are terrestrial, fresh water as well as marine. Body is covered by spirally coiled univalve (single valve shell) Head is distinct with tentacles. Foot is large, muscular and flat. Body is asymmetrical in adult due to torsion. Example Pila, Helia, Limaxetc Pelecypoda/Bivelvia They are mostly marine and few are fresh water. Body is laterally compressed and bilaterally symmetrical. Body is covered with bivalve shell. Each shell consists of two valves morably hinged dorsally. Head is absent. Eyes, tentacles and radula are absent. Foot is wedge or hatchet shaped for digging. Example Ostreaetc Cephalopoda: Foot arises from head, which is modified into ariss with suckers.