ECHINODERMATA 



251 



system of tubes filled with water which enters through the 

 sieve-like madreporic plate near the aboral pole of the animal. 

 From here a " stone canal" leads to a ring canal which circles 

 the mouth and gives off five radial canals to the five rays of the 

 body. Each radial canal has numerous lateral branches which 

 end in tubular processes of the integument, called tube feet. 



FIG. 141. Starfish, oral view showing tube feet. (From Galloway, 

 after Leuckart and Nitsche.) 



At the base of each foot there is a bladder-like lateral enlarge- 

 ment of the foot canal, the ampulla. By its contraction the 

 ampulla forces the water into the foot causing it to elongate. 

 When the foot contracts the water is forced back into the 

 ampulla. The end of the foot is provided with a sucking disc 

 by which it may be attached to an object. By this mechanism, 



