174 MANUAL OF ZOOLOGY SECT. 



industry in connection with which they are know as Beche- 

 de-mer or Trepang. A Holothurian (Fig. 91) is roughly 

 comparable to a Sea-urchin the body of which has been 

 drawn out in the direction of the line joining mouth and 

 anus, so that it has assumed a long and narrow form. But 

 there is only exceptionally a rigid shell of plates, the body- 

 wall being nearly always flexible, and sometimes quite soft 

 and supported only by scattered calcareous spicules ; and 

 usually one side, habitually directed downwards, is modified 

 as a ventral surface. A circlet of tentacles surrounds the 

 mouth. Five regular zones of tube-feet sometimes run from 

 mouth to anus : sometimes those on the dorsal surface may 

 be modified ; sometimes the tube-feet are scattered over the 

 entire surface ; and in some forms (such as the worm-like 

 Synapta and its allies) they are entirely absent. 



5. THE CRINOIDEA 



The Feather-stars and their allies, constituting the class 

 Crinoidea, bear a superficial resemblance to the Star-fishes 

 and Brittle-stars, but present some important points of 

 difference. The body of a Feather-star (Fig. 92) is star- 

 shaped, with a central disc and five arms which are bifur- 

 cate at their bases. On that surface of the disc which is 

 directed upwards in the natural position of the animal is in 

 the centre the mouth and on one side the anus. On the 

 opposite surface are attached whorls of slender curved 

 cylindrical appendages, the cirri, by means of which the 

 animal is able to anchor itself temporarily to a rock or a 

 seaweed. The arms are long, flexible and tapering and shaped 

 somewhat like a feather, with a main axis and a pair of 

 lateral rows of short slender branches, \hepinnules. The arms 

 act as the locomotive organs of the animal, their waving 



