ECHINODERMATA 215 



majority of the Holothuroids the madreporic tubes open into 

 the body cavity instead of opening to the exterior. In conse- 

 quence the fluid which is found in the water-vascular system is 

 the same as that of the body cavity and contains amoeboid cells. 

 In the crinoids also the water-vascular system communicates 

 directly with the ccelom, but there is no true madreporic canal. 

 In its stead is found a system of ciliated water-tubes in connec- 

 tion with the ring canal. Identify the elements in the water- 

 vascular system from Fig. 100. 



248. Respiration occurs in connection with the water- 

 vascular system especially in those forms in which the tentacles 

 and ambulacral feet are possessed of thin walls (holothuroids 

 and some echinoids). In the asteroids and echinoids there are 

 thin outpocketings of the body-wall, papulae or branchiae (Fig. 

 99, d.b.), the cavity in which is continuous with the body cavity. 

 The body fluids may thus be aerated from the water outside. 

 In some forms water is drawn into special branching pockets 

 (respiratory tree) in the wall of the rectum, and later is forced 

 out again. 



249. Circulation. The circulatory vessels are merely partly 

 differentiated portions of the ccelom or body cavity. Our knowl- 

 edge is by no means complete but it seems that in none of the 

 groups is there a complete separation of the blood spaces from 

 the ccelom. There are probably no contractile hearts. The 

 walls of the blood spaces may bear cilia which assist in securing 

 the motion of the fluid. The blood contains migratory cells, 

 usually colorless, and is identical with the fluid in the body 

 cavity. The general body contractions are important in caus- 

 ing motion of the fluids. It should be remembered that the 

 water- vascular system is also partly circulatory in function. 

 The blood vessels of the various classes agree in having a central 

 circular portion consisting of one or more rings, with radial 

 tubes running into the arms, and in some instances vessels which 

 accompany the intestine. The vessels of the oral surface are, 

 throughout, in close connection with the nervous epithelium 

 (Fig. 98, r.b.). 



