112 



ANIMAL BIOLOGY 



FIG. 95. "Aris- 

 totle's lantern," the 

 chewing apparatus of 

 the sea urchin. 



sort of ball-and-socket joint to rounded prominences on 



the shell, and they can be moved in different directions 



by a short ring of muscle fibers attached near the base. 

 Both spines and tube feet may be em- 

 ployed in locomotion, but the spines also 

 serve as organs of protection. After 

 removing the spines the shell of the sea 

 urchin may be seen to consist of several 

 regularly arranged series of plates. The 

 plates are perforated where the tube 

 feet are attached. The sand dollars are 

 greatly flattened sea urchins with very 

 short spines. 

 The sea cucumbers, or Holothurians, have an elongated 



body with a flexible and usually somewhat leathery wall. 



They are generally somewhat flattened on the side upon 



which they crawl, and the mouth is sur- 

 rounded by tentacles which are used in 



the capture of food. With rare excep- 

 tions, there are tube feet projecting 



through the body, and these are usually 



arranged in five double rows. Some of 



the holothurians are extensively used by 



the Chinese as food. 



The Crinoids, or sea lilies, are usually 



attached by a jointed stem, although there 



are a few species that swim freely through 



the water. The usually cup-shaped body 



bears several branching arms which are 



furnished on the upper side with grooves 



which lead to the mouth. Very numerous 



species existed in past ages of the earth's history. Their 



remains are common in the older rocks, but there are 



a comparatively few forms living to-day. 



The eggs of echinoderms are generally shed directly 



into the water where they are fertilized and develop 



FIG. 96. A cri- 

 noid or sea lily. 



