pS A Guide to the Zoological Collections in the 



Special breathing organs are not generally found in the 

 Echinodermata. The water-vascular system aids in res- 

 piration, as does also the general surface of the body-cavity 

 and its contained organs, and often also the general external 

 surface of the body. 



The Echitiodermata possess blood-vessels, in which 

 colourless blood circulates ; they are hard to trace, but 

 can be seen in the dissection of Holothuria in Case 36. 

 They also possess a nervous system, and in the Star-fishes 

 there are simple eyes, which are placed at the tips of the 

 rays, but are hard to trace. 



The Echinodermata do not usually produce offspring 

 by budding, but when an animal such as a Star-fish loses 

 one of its rays, not only can a new ray be grown from the 

 stump, but it appears that the detached ray also can often 

 by growth and budding give rise to a new Star-fish. (See 

 specimens in Case 18). 



In some Star-fishes the rays are thrown off whenever 

 the animal is disturbed, so that this regeneration of new 

 rays must be a very common event. The ordinary repro- 

 duction of new individuals, however, is always sexual — the 

 sexes being with few exceptions separate. 



The Echinodermata are divided into five great Classes 

 — (i) the Asteroidea or Star-fishes, (2) the Ophiuroidea 

 or Brittle Star-fishes, (3) the Crinoidea or Sea-lilies, (4) the 

 Echinoidea or Sea-urchins, and (5) the Holothuroidea or 

 Sea-cucumbers. 



I. ECHINODERMATA ASTEROIDEA (STAR -FISHES). 



[Casttrit Ktitll-ras£0 17-24.] 

 The Star-fishes are characterized by their pentagonal 

 or five-rayed shape. Some, however, have more than five 

 rays [e.g,^ the Acanthaster exhibited in Case 24 has twenty- 

 one, and the Brisinga in Case 23 has thirteen). On ex- 

 amining a typical Star-fish we find it to consist of a 

 central flat disk with five regular flat-pointed rays, the 

 whole being encased in a thick leathery integument hardened 



