THE STAKFISH. 271 



off the tip of a tube foot, and, by probing through it with 

 a slender bristle, determine its relation to the ampullae 

 above. Determine the relation of the ampullae to the 

 radial canal. Pull off a tube foot entire, and dissect it in 

 a drop of clove oil or glycerine, under a lens, to see the 

 delicate muscle fibers in its walls, and the oblique fibers 

 near its tip, which are inserted into the middle of the ter- 

 minal disk. How does the starfish maintain its foothold ? 



Circulatory System. Beside the stone canal, there may 

 be seen in a starfish that has been opened alive a delicate 

 pulsating tube, sometimes called the heart. The blood 

 vessels which run out from this are so minute as to be 

 traceable only by sectioning. 



Nervous System. A portion of the nervous system 

 may be seen on the oral surface without dissection, as it 

 is covered only by transparent epithelium. Find : 



1. A circumoral nerve wing. 



2. Radial nerves extending out from the latter along 

 the oral side of the radial canals to the tip of the arms. 



3. Masses of nerve tissue, called ganglia, at the junction 

 of the radial nerves with the circumoral ring. 



4. A pigmented spot, called the eye, at the distal end 

 of each radial nerve. 



The Body Wall. If almost any portion of the body 

 wall be examined in cross section, about three layers will 

 be distinguished in it. 



1. A tough outer membranous layer of epithelium 

 covering the whole animal except the spines. 



2. A middle layer of connective tissue, in which are 

 imbedded the calcareous plates which make up the hard 

 parts of the skeleton. 



3. A thin lining layer of peritoneal epithelium. This 



