RAYS AND BILATERAL SYMMETRY 257 



connection with the interradial mesenchymatous tissue and the 

 middle portion of the ray grows over beyond this point, thus 

 throwing the whole ray into almost a circle. This mode of 

 growth causes the new coelum, ccv^, to take the form of a thin, 

 rather broad, deep, curved pocket. 



Figs. II and 12, taken from frontal sections of new rays of 

 about this stage, show the relations and character of the tissues 

 of the various parts. The thick, epithelial character of both the 

 coelomic and water-vascular walls is noticeable, and at amb.m. 

 one sees that the muscular wall of the tube feet has already 

 begun to differentiate. This begins, in fact, when only five feet 

 are present. 



An interesting question, though perhaps of no great impor- 

 tance, is how the septum of the radial perihasmal canal is pro- 

 duced. We have not been able to determine this point with 

 certainty, but judging from what we can make out from our 

 sections, and from the nature of the two ring perihasmal canals 

 as this is made known to us by embryology, the process 

 seems to be as follows : The inner perihcemal, canal is, it 

 will be recalled, a single, uninterrupted canal derived from 

 the larval enterocoel as a single sac. The outer canal, on 

 the other hand, is not, in reality, a continuous canal at all, 

 but rather a series of five canals, each arising independently 

 of the others, whether from out-pocketings from the en- 

 terocoel or in part as mesenchymatous cell-aggregates that be- 

 come hollow secondarily, does not matter now. The five closed 

 sacs, as they originally are, become arranged interradially as 

 the rays of the young star take form, the two ends of each sac 

 being carried out into the two rays between which it is placed 

 to form half the double radial perihcemal canal of the respective 

 rays. The middle portion of the sac, on the contrary, remains 

 in the disc, interradially located, and becomes closely applied 

 to the inner canal along its outer side ; so that the outer canal is 

 interrupted at every radius by the base of the septum of the 

 radial canal, while the lumen of the canal that lies in any 

 given interradius is continuous with the lumena of the two 

 rays between which it is situated. The septum, then, between 

 the inner and outer canals, as also that of each radial canal, is 



