A STARFISH 137 



In studying this system find first the madreporic plate and 

 the stone canal, and trace the latter from the madreporic plate 

 to the ring canal. Remove the spines which project over the 

 peristome and find the ring canal. It is a delicate tube, of 

 about the diameter of a needle, which surrounds the mouth, 

 running around the base of the arms at the point where the 

 peristome joins them ; it is thus, like the radial canals, outside 

 the body-cavity. Remove some of the ambulacral feet from a 

 ray, and find again the delicate radial canal which lies along 

 the middle of the ambulacral groove. Trace it to the ring 

 canal. Cut the aboral body-wall from one of the bivial rays, 

 remove the liver, and observe the ampullae. Press them and 

 notice that the feet are thereby extended. 



The ambulacral system will be seen to be a system of tubes 

 extending throughout the body and in communication with the 

 sea water. They are filled with a fluid which is not, however, 

 pure sea water, but is rather a watery serum in which float 

 amoeboid cells. This fluid is driven into the tube-like ambu- 

 lacral feet, which thereby acquire rigidity and are extended. 

 The system is the locomotory system of the animal. It moves 

 by extending the feet, attaching the sucker discs at their ends 

 to some stationary object, and then drawing them in. The 

 animal is thus able to pull itself slowly along. The ambu- 

 lacral system possibly also exercises excretory and respiratory 

 functions. 



Exercise 8. Draw a diagram of the ambulacral system. 



There are no special respiratory and excretory organs. These 

 functions are exercised by the papulae and possibly the ambu- 

 lacral feet. 



The nervous system consists of a circumoral nerve ring, which 

 lies just beneath the ambulacral ring canal, and five radial nerves, 

 which proceed from it along the median line of the ambulacral 

 grooves to the tips of the arms. Each radial nerve ends with 



