Echinodermata. 183 



15. Traced to its lower end, the stone canal may be found 

 to enter a membranous hollow ring, whose outer border rests 

 against the inner surface of the hard parts surrounding the 

 mouth ; this tube is the circumoral water ring. Connected with 

 its inner surface, find several pairs of pouches, which in the 

 contracted state are mere buttonlike projections. How many 

 of these are there, and are they all in pairs ? 



Observe also the pouches, like ampullae, connected with the 

 upper part of the hard ring around the mouth. Press on the 

 water ring at the level of the peristome, and watch the effect of 

 this action on these last-named pouches or vesicles. Is there 

 any connection between them and the water ring? 



1 6. Inclosing the stone canal is a thin membrane, the peri- 

 cardium. Carefully tear it away. Alongside the stone canal is 

 a soft tube, sometimes called the " heart," but whose function is 

 doubtful. 



17. Cut across the middle of a ray in two places, about an 

 inch apart, and make a careful study of the part included 

 between the cuts. Remove the hepatic ceca, observing again how 

 they afe suspended by the mesenteries. Cut into the aboral 

 wall in the middle line and spread open the ring. Observe the 

 depressions in its inner surface ; in the bottoms of these de- 

 pressions find small holes. What is the relation between these 

 holes and the nearest structures seen on the outside ? 



1 8. Slowly peel away the thin membrane which lines the 

 interior of the ray, noting especially the connection between 

 this membrane and the depressions above noticed. Also watch 

 closely the aboral tentacles while tearing away this lining mem- 

 brane. 



19. Turn now to the outside of the ray and gently scrape 

 the surface. A thin layer here may also be easily removed. 

 Thoroughly clean a small area, noting that the aboral tentacles 

 come away with this layer. 



There will now remain a tough white layer in which are 

 embedded the calcareous plates which constitute the skeleton. 



