212 HOMOLOGIES. 



water-tubes which are their locomotive appen- 

 dages. For this reason these narrower zones are 



Portion of Sea-Urchin representing one narrow zone with a part of the broad 

 zones on either side and the ab-oral area on the summit. 



called the ambulacra, while the broader zones in- 

 tervening between them and supporting the 

 spines are called the inter ambulacra. Motion, 

 however, is not the only function of these suck- 

 ers ; they are subservient also to respiration and 

 circulation, taking in water, which is conveyed 

 through them into various parts of the body. 



The oral aperture is occupied by five sets of 

 pieces, which may be called jaws, remembering 

 always that here again this word signifies the 

 function, and not the structure usually associated 

 with the presence of jaws in the higher animals ; 

 and each of these jaws terminates with a tooth, 

 set in its centre. Even the mode of eating in 

 these animals is controlled by their radiate struc- 

 ture ; for these jaws, evenly distributed about the 



