196 



PRACTICAL ZOOLOGY 



Sea Urchins. — The common sea urchin is almost spherical 

 in shape, and covered with long spines, from among which the 

 tube feet extend (Fig. 116). It lives principally on rocky shores, 

 but it has such relatives as the sand dollars, which are flat like a 

 silver dollar and bury themselves in the sand. 



Fig. 116. 



A sea urchin. (From Clark.) 



Sea Cucumbers. — Sea cucumbers (Fig. 117) are so-called 

 because of their resemblance to the garden vegetables of that 

 name. They are not hard and spiny like their relatives, but have 

 a thick, soft body wall. The tube feet around the mouth are 

 modified as tentacles for obtaining food. 



Their food consists of organic particles extracted from the 

 sand or mud. Some species are said to stretch out their sea- 

 weedlike tentacles, on which many small organisms come to 

 rest. " When one tentacle has got a sufficient freight, it is 

 bent round and pushed into the mouth, which is closed on it. 



