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.