THE STARFISH. 157 



several pairs of pouches, which in the contracted 

 state are mere button-like 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 connec- 

 tion between them and the water-ring ? 



16. Enclosing the stone canal is a thin membrane, the 

 pericardium. Carefully tear it away. Alongside 

 the stone canal is a soft tube, the heart. In the live 

 starfish it may be seen to pulsate. 



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 

 caeca, observing again how they are suspended by the 

 mesenteries. Cut through the aboral wall in the 

 middle line and spread open the ring. Observe the 

 depressions in its inner surface; in the bottoms of 

 these depressions find small holes. What is the rela- 

 tion between these holes and the nearest structures 

 seen on the outside ? • 



18. 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 membrane. 



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

 the surface. A thin layer here may also be easily re- 

 moved. Thoroughly clean a small area, noting that 

 the aboral tentacles come away with this layer. 



