A SEA URCHIN 123 



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 madreporite. 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, at- 

 taching the sucker disks at their ends to some stationary 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. 



Respiration and excretion are performed by the peristomial 

 gills and the ambulacral feet. 



The nervous system cannot be studied in a dissection. It is 

 essentially like that of the starfish. From the ring nerve around 

 the oesophagus five radial nerves pass along the median line of 

 the rays to the ocular plates, where each terminates in a tentacle. 

 The entire system is within the body cavity instead of a portion 

 of it being outside as in the starfish. 



