derm. As will be seen later, their lunen reappear 

 and they become the definitive alimentary organs 

 of the adult ophiurid. 



Returning to the consideration of the water 

 system we find in "E", instead of one pair of ten- 

 tacles on each radial canal as in "D", there two pairs 

 present; Fig.17.tl and t2. the second pair having 

 grown out of the radial canal between the first pair 

 and the water ring. The second pair is much small- 

 er than those which were fifst to be formed, and con- 

 trary to what on© would expect, this discrepancy 

 in size does not disappear as time goes on. This 



is also true in the sea-urchins in which the pri- 



in the larva 

 inary tube feet are enormously larger than those 

 A 



which are subsequently formed. The primary tube feet 

 in this case gradually diminish in size after the 

 adult form is reached. . 



As a rule among eehinoderms the tube feet or 

 tentacles are formed centrifunally from the radial 

 canals; that is, between the end tentacle and thelast 

 pair of tube feet or tentacles Slready formed. This 

 process keeps the undifferentiated growing point 



