304 



COLLEGE ZOOLOGY 



Figure 190. Representatives of the living classes of echinoderms. The figures are not drawn 

 to scale. 



when irritated, may be opened and closed 

 by several sets of muscles. Their function is 

 to protect the dermal branchiae, to prevent 

 debris and small organisms from collecting 

 on the surface, and to capture food. The 

 rays may be flc}4,ed slowly by a few muscle 

 fibers in the body wall. The tube feet are 

 also supplied with muscle fibers. 



Water-vascular system 



The water-vascular system (Fig. 191) is a 

 division of the coelom, peculiar to echino- 



derms. Beginning with the madreporite, 

 the following structures are encountered: 

 the stone canal running downward enters 

 the ring canal, which encircles the mouth; 

 from this canal 5 radial canals, 1 in each 

 ray, pass outward just above the ambulacral 

 grooves. The radial canals give off side 

 branches from which arise the tube feet and 

 ampullae (Fig. 193). There are 9 small 

 spherical swellings on the inner wall of the 

 ring canal which open into its lumen; these 

 are called Tiedemann's bodies. No function 

 is known for these bodies. 



