234 GENERAL ZOOLOGY 



point. The breathing organs are finely branched gills, located 

 on the oral surface around the mouth. 



It feeds on both animal and plant substance. Small ani- 

 mals are captured by the tube feet. The tube feet may be 

 extended to a distance equal to half the diameter of the 

 body ; they affix by the sucking disk to some small creature 

 and draw it to the mouth. All food is then ground into bits 

 by the five sharp teeth before being swallowed. 



A sea urchin, when taken from the water and placed in a 

 person's hand, causes a gentle tickling of the skin. This it 

 does with its movable spines. The spines may be of use as 

 levers in pushing the body along, but locomotion in a defi- 

 nite direction is accomplished by the tube feet on the oral 

 surface, aided by some of those on the aboral surface, all 

 extending and contracting much as in the starfish. The 

 uppermost tube feet are used as tentacles for feeling and it 

 may be for smelling also. 



In certain parts of our eastern coast members of the 

 genus Strongylocentrotus live in cavities -in the rock which 

 are excavated by the animals themselves. A related species 

 living on the coast of California burrows a hole in the solid 

 rock deep enough to conceal itself entirely. No one knows 

 positively how the burrowing is done. Probably three fac- 

 tors take part to a varying degree, — gnawing by the teeth, 

 slow grinding by voluntary movement of the spines, and 

 incessant slight turnings of the whole body by the waves. 



The Sea Cucumber 



Sea cucumbers of various species are found in every 

 ocean. The species represented in Fig. 127 (Cucuma'ria 

 chronhjel'mi) is found in Puget Sound, Washington. It lives 

 on the bottom. 



The animals are about four inches long. The body is cylin- 



