36 



BEGINNERS' ZOOLOGY 



also called ambuiacral feet (Latin ambulacra, " forest walks "). 



There is a water holder {ampulla), or muscular water bulb at the 



base of each 

 tube foot (Fig. 

 58). These con- 

 tract and force 

 the water into 

 the tube feet and 

 extend them. 

 The cuplike 

 ends of the 

 tubes cling to 

 the ground by 

 suction. The 

 feet contain 



delicate muscles 

 by which they 

 contract and 

 shorten. Thus 

 the animal pulls 

 itself slowly 



along, hundreds of feet acting together. The tube feet, for their 



own protection, may contract and retire into the groove, the 



water which extended them being sent back into the ampulla. 



This system of water 



vessels (or water- 

 vascular system) of 



the echinodermata 



is characteristic of 



them ; i.e. is not 



found elsewhere in 



the animal kingdom. 



The grooves and the 



plates on each side 



of them occupy the 



ambuiacral areas. The rows of spines on each side of the grooves 



are freely movable. (What advantage ?) The spines on the aboral 



surface are not freely movable. 



FlG. 57. — Starfish, from below; tube feet extended. 



4 r% 



Fig. 58. — Section of ONE rav and central portion 

 of starfish. 



fit fit fit '"be f"t more or less extended; a«, eye spot; 

 /■.gills; <rVi, stomach; m, madreporite; st, stone canal; 

 p, ampulla; ei, ovary. 



