ii8 PHYLUM ECHINODERMATA 



bulacral feet from a ray and find again the delicate radial canal 

 which Hes 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 pure sea 

 water, but is rather a watery serum in which float amoeboid cells. 

 This fluid is driven into the tubelike ambulacral 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 disks at their ends to some stationary 

 object, and then drawing them in. The animal is thus able to 

 pull itself slowly along. The ambulacral system possibly also 

 exercises excretory and respiratory functions. 



Exercise 7. 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 ambula- 

 cral 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, tlach radial nerve ends with a 

 pigment eye. There are no other organs of special sense. The 

 main nerves of the starfish do not lie within the body cavity but 

 in the integument and can thus be seen from the outside. There 

 are, however, in addition to these nerves, other less important 

 ones which are internal. We have already observed the radial 

 nerves in the median line of the ambulacral grooves; the ring 

 nerve can also be seen as a slight ridge just beneath the ring canal. 



Exercise 8. Draw a diagram representing the nervous system. 



Exercise 9. Draw a diagram representing a vertical section of the 

 animal passing through the madreporite and the anterior ray 

 (that is, the middle trivial ray). 



