66 



INTRODUCTION TO ZOOLOGY 



prolongation of the digestive system, all the chief organs of 



the body being confined 

 to the central disc. The 

 arms are very flexible 

 and yet at the same 

 time brittle; the animal 

 readily snaps them off if 

 annoyed. A brittle-star 

 is altogether a much 

 more active creature 

 than a starfish, moving, 

 not by means of its tube- 

 feet, which in Ophiuroids 

 have probably a respir- 

 atory function, but by 

 the muscular move- 

 FiG. 34.-Common Brittle-star °ients of its very active 



{Ophiothrixfragais). arms or rays. 



Sea-urchins. 



Class III.: ECHINOIDEA 



Sea-urchins, or Echinoids, occur plentifully on 

 our coasts, and the small purple-tipped urchin 

 (Echinus miliaris) can easily be kept alive in a sea-water tank. 

 In these forms the body is no longer rayed, but is apple- 

 shaped, varying in horizontal diameter from several inches, in 

 the case of the Common Urchin, to about one inch in the little 

 purple-tipped species. 



In all Sea-urchins the body is covered with a continuous 

 hard shell, except where soft skin surrounds the mouth. This 

 shell is covered with many movable spines, and on these, 

 with the aid of some of the five double rows of beautiful 

 tube -feet, the Urchin crawls along the sea-bottom, mouth 

 downwards. The mouth contains a complicated tooth-bearing 

 structure consisting mainly of five chisel-edged teeth which, 

 together with the calcareous ossicles which support them and 

 the muscles which move them, form the structure known as 

 Aristotle's lantern. Owing to the extensible soft membrane that 

 surrounds the mouth, the "Lantern " can be projected beyond 

 it, and so the teeth can be given free play to crop the sea- 

 weed which, together with some animal matter, forms the 



