PHYLUM  MOLLUSCA

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.

Brief History

Aristotle  described  a number of molluscs. Johnston (1650)  proposed the  name  of the phylum.

Important  Characters

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 .