Phylum Mollusca

Phylum Mollusca Mollusca is an ancient group of animals which have lived on earth since the last 500 million years . It is the second largest phylum and includes more than 600,000 species. They are soft bodied animals. General characters: 1. Mollusca are essentially aquatic, mostly marine, few fresh water and some terrestrial forms. 2. The body is soft, unsegmented, triploblastic, bilaterally symmetrical, and consist of head, foot, mantle, and visceral mass. 3. The body is clothed with a one layered often ciliated epidermis. 4. Body is commonly protected by an exoskeletal calcareous shell of one or more pieces, secreted by the mantle. 5. Head is distinct, bearing the mouth and provided with eyes, tentacles and other sense organs. 6. Ventral body wall is modified into a muscular flat or plough like surface, the foot which is variously modified for creeping,burrowing and swimming. 7. Mantle is a fold of body wall that leaves between itself and the main bod mass, the mantle cavity. 8. Visceral mass contains the vital organs of the body in a compact form taking the form of a dorsal hump or dome. 9. Body cavity is haemocoel; the true coelom is generally limited to the pericardial cavity and the lumen of the gonads and nephridia. 10. Digestive tract is simple with an anterior mouth and posterior anus. 11. Pharynx contains a rasping organ the radula. 12. Circulatory system is open except in cephalopods which show some tendency towards a closed system. 13. Respiratory system consists of numerous gills or ctenidia usually provided with osphradium at the base. Lung is developed in terrestrial forms. Respiratory pigment is haemocyanine. 14. Excretory system consists of a pair of metanephridia. 16. Sexes are separate but some are hermaphrodite. 17. Fertilization is external or internal. 16. Development is either direct or with metamorphosis.