ANIMAL BIOLOGY 



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

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



on knobs, and the joint resembles a 

 ball-and-socket joint, and allows a 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 than the other species. 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. 



Both starfish and sea urchin rest on 

 the flattened lower surface of the body, 

 while the tube feet are stretching for- 

 ward for another step. 



OTHER ECHINODERMS 

 The sea cucumbers, or holothurians, re- 



semble the sea urchin in many respects, ^^^ A ^^-~ : - 1:"^ 



FIG. 64. --THE SEA OT- 

 TER, an urchin with 

 mouth (<?) and vent (A) 

 on same side of body. 





SP^' ' 



FIG. 65. SEA CUCUMBERS. 



but their bodies are elon- 

 gated, and the limy plates 

 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. 



