Definition and Introduction
Basically bilateral and protostomial eucoelomate eumetazoans whose soft body (L. Millis or molluscum= soft) is unsegmented and enclosed within a skin fold ( mantle) which usually secretes a calcareous shell.
About 80,000 existing and 35,000 extinct species known .Aquatic or terres trial. Snails and mussels found along banks of freshwater bodies and the oysters shanks ( whose conch shell is commonly blown in religious ceremonies )and cowries of the sea are common molluscs .Malacology or conchology is the branch of study of mollusc and their shells.
Molluscs are of some economic important ,man uses some as food shells of some are used for making buttons beads etc.
Aristotle described a number of molluscs. Johnston (1650) proposed the name of the phylum.
1. Mostly marine, some freshwater , some terrestrial in damp soil. Creeping , free swimming , burrowing or adhered to rocks .
2. Body soft and slimy due to excessive mucilage.
3. Body basically bilateral, but may secondarily become coiled and asymmetrical due to torsion.
4. Triploblastic , unsegmented body covered by a dorsal skin fold called mantle, which often secretes a calcareous shell. Body surface usually ciliated , Space between mantle and body mass ( viscera ) is called mantle cavity.
5. Majority with a distinct head and a ventral muscular foot or podium adapted for creeping , swimming or burrowing.
6. Coelom reduced, restricted to pericardial cavity and to small spaces within excretory and reproductive organs .
7. Digestive vascular, excretory and nervous system well developed.
8. Digestive system with excretory glands and a liver ,Many possess a specialized rasping organ, the reproductive organs. The radula , in the their buccal cavity.
9. Open blood vascular system .Blood colourless or coloured blue or green due to the presence of a respiratory pigment , the haemocyanin.
10. Respiration mostly by paired gills (ctenidia) present in mantle cavity.
11. Excretion by paired metanehridia.
12. Nervous system consisits of a few paired gandlia and their connectives mind nerves.
Majority unisexual. Development direct. Or with some larval stage which may be veliger, trochophore glochidium .