THE STARFISH 29 



while in younger individuals they are not more than half 

 an inch long. HW f^^^Ots ^uft ORfcrftMS faL A H ? 

 Make a drawing ohowing thcGa-s 



The nervous system. Part the rows of tube f eek along 

 th&^middle of each ray and note a yellowish, white ridge, 

 the nerve of the ray. Trace this nerve to the mouth and 

 find the nerve ring that surrounds the mouth. Again, trace 

 the nerve of the ray to its outer extremity and find a red 

 spot, the eye-spot. The eye-spot is situated at the base of 

 a so-called tentacle which resembles a tube foot, but is 

 smaller and has no sucking disk. 



Diagram the nervous system. 



The water-vascular system. If a fresh specimen can be 

 obtained, cut off the tip of a ray. Note the small tube that 

 runs along the floor of the ray. With a fine pointed syringe, 

 inject some carmine into this tube. The vessels connected 

 with it can then be easily traced. Many of them can be 

 made out with an alcoholic specimen not injected. Note 

 that just inside the mouth, around the gullet, is a five-sided 

 canal, the ring canal. A hard tube runs from this to the 

 madreporic plate, called the stone canal, because its walls 

 contain a series of calcareous rings. From the ring canal 

 radiate five canals, one for each arm. These are the radial 

 canals. Their cut ends may be seen at the tip of the cut-off 

 rays. The tube feet are connected to the radial canals by 

 short tubes running to the right and left. This whole sys- 

 tem of tubes is called the water-vascular system. (See text 

 for manner of locomotion by means of this system.) 



Make a diagram of the water- vascular system. 



C. TOPICS FOR FURTHER STUDY. Compare several 

 starfish, noting the differences in size and shape of the arms 

 and central disk. Compare with the brittle stars and sea 



