208 



ZOOLOGY. 



cavity instead of opening to the exterior. In consequence 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 con- 

 nection with the ring canal. Identify the elements in the 

 water-vascular system from Fig. 98. 



242. Respiration occurs in connection with the water- 

 vascular system especially in those forms in which the ten- 

 tacles and ambulacral feet are possessed of thin walls (holo- 

 thuroids and some echinoids). In the asteroids and echinoids 



FIG. 97. 



r. p.... 



FIG. 97. Transverse section of the arm of a Starfish near the disc. Diagrammatic. 

 Lettering as in preceding figure, a.r., ambulacral rafter (ossicle*) ; ov., ovary, con- 

 taining ova. 



Questions on Figs. 96 and 97. What are the principal sets of organs 

 represented in the disc of the starfish? Which of these have radial por-? 

 tions going into the arms? Follow carefully the ectodermal, entodermal 

 and mesodermal boundaries. Locate and identify the various structures 

 lettered, and determine as far as possible, whether the essential part of 

 each is furnished by ectoderm, entoderm or mesoderm. Is there a coelom? 

 Your evidences? What is the relation of the water- vascular cavity to the 

 coelom, in origin? 



