A STARFISH 147 
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 ampulle. 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 
amceboid 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 papule 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 
