A SEA URCHIN 153 



The ambulacral system is similar to that of the starfish. A ring 

 canal surrounds the oesophagus just inside the inner end of the 

 dentary apparatus and is connected with the madreporic plate 

 by means of the stone canal. This organ is a small tube which 

 lies in contact with the oesophagus and also, in the neighbor- 

 hood of the aboral body-wall, with the intestine. From the 

 ring canal five radial canals pass along the median lines of the 

 rays to their aboral ends, sending off branches to the ambulacral 

 feet. The entire system of tubes, except the ambulacral feet, 

 is within the body-cavity, instead of outside of it, as in the star- 

 fish. Look on the inner surface of a ray for the radial canal. 

 On each side of it observe the row of small vesicles, the ampullae, 

 the reservoirs of the ambulacral feet. Determine the exact 

 relation of the ampullae to the feet, and of both to the ambu- 

 lacral pores in the shell. It will be seen that there are two 

 rows of these latter on each side of the radial canal. Through 

 one of these rows the branch canals pass from the radial canal 

 to the ambulacral feet on the outside of the shell ; through the 

 other row projections of the feet pass back into the body-cavity, 

 where they expand to form the ampullae. There is thus a single 

 row of feet on each side of the radial canal in each ray. 



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

 tubes extending throughout the body, and communicating with 

 the sea water through the madreporic plate. It is filled with a 

 fluid which, as in the starfish, is not pure sea water, but is 

 rather a watery serum in which float amoeboid cells. This fluid 

 can be driven into the ambulacral feet, which acquire rigidity 

 and are thereby extended. The animal moves by extending 

 the feet, attaching the sucker discs at their ends to some sta- 

 tionary object, and then drawing them in; it is able thus to 

 pull itself slowly along. Some sea urchins with long spines 

 also move on the tips of their oral spines as on stilts. 



Exercise 8. Draw a diagram representing the ambulacral system. 



