92 



STARFISH, SEA URCHIN, BRITTLE STARS 



and wrapped around the victim, which is then dissolved 

 and digested, after which the stomach is withdrawn into 

 the body. 



The water vascular system. — The madreporite, of which 

 we have already spoken, is the cover to a short canal, the 

 stone canal, that runs downward and opens into a ringlike 

 canal that encircles the mouth. Five long canals, one for 



each arm, radiate 

 from the ring canal 

 about the mouth. 

 Each of these long 

 radiating canals is 

 provided with many 

 short side branches 

 to which are at- 

 tached the tube feet 

 (Fig. 45). Each tube 

 foot is connected, by 

 a slender tube that 

 passes through a 

 minute pore between 

 the ossicles, with a water bulb in the body cavity. The 

 bulb and the tube foot are both contractile. 



This whole system of tubes is called the water vascular 

 system (Fig. 45). The tubes are filled with a watery fluid 

 composed largely of sea water that flows in through the 

 madreporite. 



Locomotion of the starfish. — When the starfish desires 

 to move, it compresses the water bulb connected with each 

 tube foot and forces the water out into the latter organ. 

 This greatly elongates each tube foot which is then directed 

 forward in the direction the animal wishes to go. After 



Fig. 45. — -Diagram of the water vascular sys- 

 tem of a starfish : S, stone canal ; C, circular 

 canal ; R, radial canal ; T, tube feet ; A, am- 

 pullae. After Brooks. 



