270 ECHINODERMS. 



alimentary canal attached at its minute anal opening on 

 the aboral surface. 



The Digestive System. With the aboral crust properly 

 removed, the digestive organs will be fully disclosed. 

 Oral and aboral openings of the alimentary canal have 

 been noted. By probing with a stiff wax-tipped bristle 

 through the mouth, learn how restricted are esophageal 

 and intestinal areas of the alimentary canal, and how very 

 capacious and wide is the stomach. In the latter find : 



1. A loose, baggy oral portion which may be protruded 

 through the mouth, and which has muscles for its re- 

 traction. In feeding, the starfish commonly protrudes 

 this portion out of its mouth and in between the valves 

 of the shell, and digests the oyster there. 



2. An aboral portion of more definite shape, with which 

 the hepatic caeca communicate. Probe the ca3ca. 



The Water- Vascular System. Study this in an injected 

 specimen. 1 Observe : 



1. A vessel extending downward from the madreporite 

 beside the stomach toward the mouth. This is the stone 

 canal, so called because calcareous matter is found in its 

 walls. 



2. Circumoral canal, into which the stone canal opens. 



3. A series of radial canals extending outward from 

 the circumoral canal, lodged deep in the ambulacra] groove 

 on the underside of each arm. 



4. A double row of round water sacs called ampullae 

 within each arm on its floor on either side of a median ridge. 



5. Two double rows of ambulacra, or tube feet, in cor- 

 responding position on the oral surface of each ray. Snip 



1 The water-vascular system may be injected by inserting a cannula 

 into each radial canal near the tip of the arm, and injecting toward the 

 disk, forcing the water out at the madreporite. 



