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ESSENTIALS OF ZOOLOGY 



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 

 foot. The ampulla is bulblike and is located above the roof of the 



Radial canal 



Madreporite 

 Stone canal 



Tiedemann's body 



Ci ream -or a 

 canal 



Tube foot 



Fig. 138. — Diagram of water-vascular system of the starfish. 



ambulacral groove in the coelom. It is connected through its 

 ambulacral pore with the contractile tube foot which hangs down 

 into the ambulacral groove. The distal or free end of the foot 

 has a slightly inverted, suckerlike shape. The proximal pair of 

 ampullae in each arm of some starfish lack the tube feet and are 

 sometimes erroneously called Polian vesicles. Alternate tube feet 

 are farther from the radial canal than the others on each side. The 

 ampullae and tube feet function effectively in locomotion, the am- 

 pullae contracting to force water into their respective tube feet 

 to extend them. The walls of both ampullae and tube feet are mus- 

 cular. In large starfish the tube feet may be extended an inch or 

 two. The sucker ends of these tube feet work like a vacuum cup 



