ECHINODERMATA 



151 



>roximate area of the bottom of all the little tube feet 

 involved with a square inch and multiplying the re- 

 mit by fifteen, for every square inch of surface bears 

 le weight of fifteen pounds of air. 



The tube feet are able to stiffen because of their con- 

 Lection with a system of tubes in the animal. Sur- 

 rounding the oesophagus is 



tubular ring which gives 

 )ff a blind tube in every 

 •ay and in addition a tube 

 r ith calcareous walls that 

 >pens to the outside on the 

 tarsal side in a perforated 

 )late. Each radial canal 

 fives off at regular in- 

 tervals pairs of canals 

 which connect with the 

 tube feet. These are hol- 

 low muscular tubes each 

 of which ends inside the 

 ray in a little sac and out- 

 side the ray in a sucker 

 (Fig. 85). The system is filled with fluid. If the 

 sacs contract fluid is forced into the feet and they 

 stiffen, if the feet contract the fluid is forced back into 

 the sac and the feet are withdrawn. 



Assimilation. — Assimilation takes place through the 

 respiratory system and the alimentary canal. Special 

 organs for the absorption of oxygen are developed on 

 the dorsal surface in the form of gill-like outgrowths. 

 Water passes over them and the oxygen it contains 



Fig. 85. — Water-vascular system of 

 starfish, a, ampullae or sacs ; ab, tube 

 feet ; c, radial canal ; m, perforated 

 plate ; n, radial nerve ; r, ring canal ; 

 8, stone canal. (From Hertwig.) 



