388 



Dr. J. E. Duerdcn on the 



stage each would preserve the hexameral character, but 

 would differ from the original polyp in having only one 

 pair of directives. The third order mesenteries of fig. 1 

 would be second order mesenteries of the two new polyps, 

 and the original fourth order pairs would advance to members 

 of the third order. 



Other polyps which have been examined at the bioral 

 stage reveal tiiat the arrangement of the mesenteries is not 

 always so regular as that here given. Yet the essential facts 

 are invariably the same, namely, the insertion of one pair of 

 directives and a certain number of pairs of the original 

 complete mesenteries on each stomodeeum and the entocoelic 

 plane of separation of the two moieties. 



As the bioral polyp continues its growth more pairs of 

 mesenteries extend as far as each stomodseum, and other new 

 pairs arise on the polypal wall in a more or less irregular 

 manner. 



The next fission-stage available is one in which four oral 

 apertures are present on an elongated disk surrounded by a 

 single system of tentacles and a single column-wall. Stages 

 with three oral apertures seem to be very rare, so that 

 evidently fission of each of the two stomodaa occurs at 

 about the same time. Fig. 3 represents the essential rela- 

 tionships of the mesenteries and the four stomoda^a. By 



Fig. 3. 



Diagrammatic arrangement of the mesenteries in a polyp of Manicina 

 areolata with four stomodseal systems. 



this time scarcely any constancy is evident in the number of 

 mesenteries attached to each stomodfeura or in the number of 

 incomplete pairs which may occur in the exocoele between 

 any two complete pairs. 



Only the two primary pairs of directives still occur, situated 



