They are exclusively present in marine water and there are no parasitic forms. Similarities with chordates :

(i) They have tube within tube type of body plan which has evolved along deuterostomic evolutionary line.

(ii) They possess a true coelom called enterocoelom.

(iii) They have mesodermal skeleton made of calcareous plates or ossicles.

All these characters make the echinoderms closer to chordates. The other characters are :

1. The symmetry is bilateral in larvae but pentamerous radial in adults.

2. Many echinoderms bear a number of calcareous spines on their body surfaces called tubercles. Between the spines, there are pincer like structures called pedicellariae to keep the surface clean. Pedicellariae are made of three calcareous plates. Two calcareous valve like structures in the form of jaws resting upon a basal calcareous plate.

3. Between the spines, there are finger like processes called dermal branchiae which help in respiration.

4. Mouth is on ventral/oral side and aniss is on dorsal or aboral side.

5. There is no distinct anterior and posterior end i.e., no cephalisation.

6. Haemal and Perihaemal Systems: Instead of blood vascular system, there are present haemal and perihaemal systems which are of coelomic origin. Thus the so called circulatory system is open type. The so called blood is often without a respiratory pigment. There is no heart.

7. Respiratory Organs : Gaseous exchange occurs by dermal branchiae or papulae in star fishes, peristominal gills in sea urchins, genital bursae in brittle stars and cloacal respiratory trees in holothurians. Exchange of gases also takes place through tube feet.

8. Excretory organs : Specialized excretory organs are absent. Nitrogenous wastes are diffused out via gills or dermal branchiae. Ammonia is chief excretory matter.

9. Sexes are separate. Reproduction is sexual. Fertilization is usually external. Development is indirect with free-swimming larva.


Concept Builder

1. The starfish uses the suction cup-like ends of the tube feet of one arm to hold on to rocks, as it moves the other arms. It can open bivalves (molluscs) by attaching two arms to either sides of a bivalve and pulling them apart. The bivalve opens out, the stomach of starfish is then everted through its mouth and introduced into the open mollusc which is partly digested before being ingested. Echinoderms predate on coral polyps also.

2. Word roots and origin : ampulla from Latin meaning ''flask''.