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TEXTBOOK OF ZOOLOGY 



over it. The tube feet are attached to the shells, and, by coopera- 

 tive activity, an enormous pull is exerted on the valves of the 

 shell. After the shell is open, the stomach of the starfish is everted 

 through its mouth and is spread over the tissues of the prey which 

 are digested in situ. An abundance of digestive fluid secreted over 

 the food causes the mollusk to be digested in its own shell and this 

 material is then taken into the stomach of the starfish. It is reported 

 that between four and five dozen clams may be eaten by a single star- 

 fish in a week. It has also been shown that a starfish may survive 

 after months of fasting. After feeding, the stomach is withdrawn 

 into the body cavity by five pairs of retractor muscles, one pair ex- 

 tending from the pyloric portion to the ambulacral skeleton of each 

 arm. The branched, treelike gonads fill the remaining space in each 

 arm and the external 'pores from them are located in the crevice be- 

 tween adjacent arms. 



Fig. 125. — Longitudinal section through the central disc and one ray of a star- 

 fish, a, anus ; am, ampulla ; car, cardiac stomach ; coe, perivisceral coelom ; ey, 

 eyespot ; hca, hepatic caeca ; i, intestine ; m, mouth ; tnp. madreporic plate ; nr. 

 nerve ring ; oe, esopliagus ; os, ambulacral ossicle ; py, pyloric sac ; ra, radial 

 canal ; re, ring canal ; rca, lectal caeca ; sc, stone canal ; sp, spine ; tf, tube feet. 

 (From White, General Biology.) 



The water-vascular system is composed of the madreporite, stone 

 canal, circumoral or ring canal, radial canals, Tiedemann's bodies, 

 lateral canals, ampullae, and tube feet. Water is taken in through 

 the sievelike madreporite on the aboral side of the central disc and 

 is conducted by the S-shaped, calcareous stone canal (hydrophoric 

 canal) to the ring canal, which encircles the mouth. The movement 

 of the water through the madreporite and stone canal is accomplished 

 by the action of cilia, which line them. On the medial surface of 

 the ring canal are nine small Tiedemann's (racemose) bodies, the 

 stone canal joining the ring canal where the tenth might be expected. 

 These bodies produce amoeboid cells. The five radial canals extend 

 distally, one in the roof of the ambulacral groove of each ray. 

 Numerous paired lateral canals arise along the length of each radial 

 canal. Each ends shortly by connecting with its ampulla and tube 



