THE PHYLUM ECHINODERMATA 



in an "anterior" direction, along the ambulacral area opposite the anus. 

 The fossil record and the life cycles of these bilateral urchins indicate that 

 they have descended from ancestors which were circular and radial like 

 Arbacia. The existence of fossil forms with irregularly arranged plates sug- 

 gests, in turn, that the Arbacia type, with 20 rows regularly arranged, arose 

 from ancestors without this skeletal regularity. 



The Class Holothuroidea 



The Sea Cucumber. Thyone bnareus, a sea cucumber common along the 

 Atlantic coast from Cape Cod southward, is an example of the holothurian 

 type of echinoderm (Fig. 16.12). Fundamentally, it is radially symmetrical, 

 but the characteristic elongation of the body between oral and aboral ends, 

 and certain other specializations, give it a bilateral and often worm-like 

 appearance. The texture of the body is very different from that found in the 

 starfish and sea urchin: the expanded Thyone is soft, like a bladder partly 

 distended with fluid, and there is no skeleton except minute calcareous 

 spicules embedded in the body wall and a few larger plates in the oral 

 region. At one end is the mouth, surrounded by ten branched tentacles, and 

 at the other is the anus. The tube feet are not in distinct rows but lie 

 scattered all over the body, although internally they are connected with five 



Fig. 16.12. A sea cucumber, Thyone bnareus. In this individual the feeding tentacles are 

 well extended; note also the sand grains adhering to the tube feet, scattered irregularly over 

 the surface of the body. (Photograph by George Lower.) 



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