40 



ANIMAL BIOLOGY 



The largest species of sea urchins are almost as big as a 



child's head, but this size is unusual. The spines are mounted 

 B on knobs, and the joint resembles a 



ball-and-socket joint, and allow as wide 

 range of movement. Some sea urchins 

 live on sandy shores, other species live 

 upon the rocks. The sand dollars are 

 lighter colored. (Why?) They are usu- 

 ally flatter and have lighter, thinner 

 walls, for there is danger of sinking 

 into the sand. The five-holed sand 

 cake or sand dollar has its weight still 

 further diminished by the holes, which 

 also allow it to rise more easily through 

 the water. The flattened lower surface 



of both starfish and sea urchin causes 



the body to remain still while the tube 



feet are stretching forward for another 



step. 



Other Echinoderms 



The sea cucumbers, or holothurians, re- 

 semble the sea urchin in many respects, 



Fig. 64. — The Sea Ot- 

 ter, an urchin with 

 mouth (p) and vent {A) 

 on same side of body. 



•sSS^iil', 



Fig. 65. — Sea Cucumbers. 



but their bodies are elon- 

 gated, and the limy plates 

 very 



are absent or very mi- 

 nute. The mouth is sur- 

 rounded by tentacles (Fig. 

 65). 



The brittle stars resem- 

 ble the starfish in form, 

 but their rays are very 

 slender, more distinct 



from the disk, and the tube feet are on the edges of the rays, not 



under them (Fig. 66). 





Fig. 66. — A Brittle Star. 



