RAYS AND BILATERAL SYMMETRY 261 



ambulacral I, so that when the two systems unite to produce 

 the complete rays of the adult star they do so in this wise : 

 I, II, III, IV, V.' 

 5» I. 2, 3, 4. 



In other words, interradius 5-1 corresponds to I-II, as we are 

 enumerating the ra3'-s of the adult star. (Compare fig. 4 with 

 fig. 8 copied from Ludwig.) 



All the work that has touched the point since Bury's sum- 

 mary (Bury, '95 ; MacBride, '96 ; Goto, '97 and '99) has con- 

 firmed this result, so far as the closure of the hydrocoel ring is 

 concerned. 



The evidence bearing upon the subject is, then, to the effect 

 that the \.\no -points at which new arms are budded out in Pycno- 

 ■podia correspond to the positions of the two ends of the larval 

 hydrocoel immediately before and at the time of its closure to 

 fortn a complete ring. 



Remembering now the fact that the best evidence we have 

 indicates that the water vascular ring initiates the process in 

 the production of the new ray, the supposition is warranted 

 that the multiplication of rays in Pycnopodia is 7tierely a 

 continuation of the process that gives origin to the original 

 five. 



As to the order of formation of the five primary tentacles 

 of the larval hydrocoel, a matter that would be so significant 

 for the present question, we have but little definite information. 

 A. A.gassiz, '77, p. 32, has a statement that appears, if we in- 

 terpret it correctly, to indicate that the first folds, as he terms 

 them, for the hydrocoel rays are formed about in the region of 

 rays 2 and 3. 



Goto, '97, states that in Asterias pallida " No. 2 is the first 

 developed and is situated hindmost, i and 3 are formed almost 

 simultaneously, then 4, and lastly 5 " (p. 254) ; so that, bearing 

 in mind the relations which the primary hydrocoel rays have to 

 the completed rays, it is seen that the process of ray formation 

 for the original five would lead off with III, and terminate with 

 I, which would, in so far, be in harmony with the supposition 



' Bury states that in Bipinnaria asterigera the usual rotation does not take 

 place, and that corresponding members unite. 



