MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 463 



So far the series serves to demonstrate the important fact, lirsi suggested by Cladocora, 

 that in the establishment of the third order of mesenteries only a single pair l i i ■ — t arises within 

 each of the six primary systems, not two pairs one in the exoccele on each side of the second- 

 cycle pair— as might have been expected. Further, the pairs do not appear simultaneously, any 

 more than tl<> the members of the tirst and second cycles. They present evidence of a general, 

 though not rigid, succession from one holder of the polyp to the other. What this aspect is, 

 whether dorsal or ventral, can not be determined in polyps at this late stage, seeing that the 

 protocnemes are all complete. In isolated polyps apparently no means is available for such an 

 important determination; the relative sizes and vertical extent of the second-cycle mesenteries 

 are of no assistance. The latter are now practically of the same size, and any variation they 

 may present is of very uncertain value. 1 lowever, as in most species the succession of the .second- 

 cycle mesenteries is found to be from the dorsal to the ventral aspect, 1 have disposed the figures 

 in such a way that they indicate a like succession for the first six members of the third cycle. 



m I m 



nr 



IT 



Fig. 8g. 

 Astranffia solitaria.— An additional pair of tertiary mesenteries has appeared within the right middle exoccele (<•/, fig. 43, PI. V). 



In some instances (tig. 8c) the growth is more rapid on one side than on the other, and in fig. 8e 

 the right middle pair has lagged behind. In A. solitaria, at any rate, a certain amount of 

 individuality in growth is exhibited by each sextant, and mesenteries may appear in one irrespective 

 of the condition in other divisions. 



Clearly, in order to complete the third cycle of mesenteries according to the hexameral plan, 

 a second pair of mesenteries must now arise in each of the six primary systems, and within the 

 exoeade on the ventral aspect of each of the second-cycle mesenteries. 



Such has already taken place in tig. 8g in connection with the right middle system, but a 

 lagging behind occurs in the left ventral system, as only a single mesenterial pair is yet developed. 



Polyps of Asbrangia solitaria rarely exhibit more than seven or eight third-cycle mesenteries; 

 no specimen with the full twelve pairs has been met with. The further stages necessary to 

 complete the third order may, however, he obtained from the larger polyps of the closely allied 

 Phyllangia <tin- ricana. 



