Echinoderms

Abstract

Eucoelomata in which:-

• 1. There is evidence of metameric segmentation in the early larva though this is completely obscured in the adult. The body of the adult is secondarily radially symmetrical (usually pentaradiate) being developed from a bilaterally symmetrical larva.

• 2. The adult body has no head region but in some groups is differentiated into a central disc and radiating ‘arms’.

• 3. The gut is very variable in form and may have no anus. There are no lateral openings or pouches to the fore gut (gill slits or pouches).

• 4. The nervous system is well developed but remains largely in contact with the ectoderm.

• 5. Organs of special sense are typically present but poorly developed.

• 6. The body wall comprises a ciliated epidermis overlying the calcareous endoskeleton.

• 7. Appendages are represented by modified spines and tube feet.

• 8. Body wall musculature is weakly developed.

• 9. There is a spacious coelom which forms the body cavity and which arises in three distinct regions. The middle left coelom gives rise to the complex water vascular system.

• 10. There are no special excretory organs.

• 11. The blood vascular system is represented by strands of lacunar tissue known as the haemal system.

• 12. The products of the gonads are passed to the exterior through gonoducts.

• 13. The sexes are separate.

• 14. There are free swimming larvae of various forms, each associated with a particular class. All are essentially ‘pluteoid’ in type with the main ciliary band peri-oral.