248 



GENERAL ZOOLOGY 



spines fit, and form a ball-and-socket joint. On each tube- 

 foot area at the point nearest the center of the aboral surface 

 lies an eye ; and between every two eyes there is an opening 

 for the eggs, or spermatozoa, to emerge. A sieve-plate lies 

 in one of the spaces between two eyes. The mouth has five 

 sharp teeth which meet together in a point. The breathing 

 organs are located on the oral surface around the mouth. 



Considering the 

 structure of the 

 animal, one could 

 be almost posi- 

 tive that the sea- 

 urchin does not 

 get its food after 

 the manner of the 

 starfish. It feeds 

 on both animal 

 and plant sub- 

 stance. 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 spir>es. The spines may be of use 

 as levers in pushing the body along, but locomotion in a 

 definite direction is accomplished by the tube-feet on the 



FIG. 124. Photograph of Sea-Urchin. Slightly 

 reduced 



