208 



METAZOAN PHYLA 



small calcareous plates. The madreporite is internal. Tube feet are 

 present and serve as organs for clinging and for locomotion. 



One type of sea cucumber is represented by those which conceal 

 themselves in the crevices between rocks and which have the tube feet 

 all around the body in five double rows. Some of the tube feet adjacent 

 to the mouth, as well as the tentacles, are used in procuring food. A 

 cloaca is present in a typical sea cucumber and contains the openings of 

 two long branched tubes, the respiratory trees (Fig. 122). Respiration 

 occurs in these as well as through the cloacal wall and the walls of the 

 tentacles and tube feet. The respiratory trees also serve as excretory 



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Fig. 120. — Internal structure of a sea urchin. {From Ddage and Herouard, " Traite de 

 Zoologie Concrete," after Milne Edwards.) The oral wall of the shell has been removed and 

 the contents of the body are viewed from the oral pole with the Aristotle's lantern and 

 esophagus turned to the left. 



organs. The madreporite takes water in from the coelomic cavity. 

 Other sea cucumbers possess tube feet on only one side of the body and 

 travel about on that side, looking like huge caterpillars. Still others 

 burrow in the mud like earthworms and have no tube feet at all; they 

 seek their food at the surface of the mud and secure small living plants 

 and animals by means of their tentacles. The last named are the most 

 primitive of echinoderms. 



The holothurians exhibit a remarkable form of autotomy and regenera- 

 tion. When irritated the whole alimentary canal and the respiratory 

 trees may be thrown out through the mouth, there being developed from 

 the lower branches of the latter a mass of white tough threads in which a 



