Molluscs

Abstract

Eucoelomata in which:- 1. The body shows little evidence of segmentation. 2. A distinct head is usually present and, characteristically, the body also has a ventral muscular foot and a dorsal visceral mass. 3. The gut is liable to torsion and coiling and the anus may be directed forward. 4. The nervous system typically comprises a circum-oesophageal ring frequently concentrated into cerebral and pleural ganglia. Pedal and visceral ganglia are variously joined to this ring. 5. Organs of special sense are present and may be very elaborate. 6. The epidermis does not secrete a cuticle, but the dorsal surface is often extended beyond the body to form a mantle which typically secretes a protective calcareous shell. 7. The appendages, when present, are never jointed. 8. The muscular system is concentrated in the foot region. 9. The coelom is typically restricted to the pericardium and the cavities of the excretory and reproductive organs. In addition, an extensive haemocoel may or may not occur. 10. The excretory organ is the kidney. 11. There is a well-defined blood vascular system which is partly open, i.e. there is a heart, arteries and veins communicating with the haemocoel. 12. The products of the gonads are passed to the exterior through coelomoducts. 13. The sexes may or may not be separate. 14. In most forms the egg develops into a trochophore larva which passes through a veliger stage to the adult.