.MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 463 



So far the series serves to demonstrate the important fact, first suggested t)y Oladoan'a, 

 that in the establishment of the third oi'der of mesenteries only a single pair lirst arises within 

 each of the six primary sj'stems, not two pairs — one in the exocoele on each side of the second- 

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

 more than do the members of the first and second cycles. They present evidence of a general, 

 though not rigid, succession from one border of the polyj) 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. However, as in most species the succession of the second- 

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

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



m -^ HI 



m 



IT 



Fig. ig. 

 Astrangia mliiaria. — Au additional pair of tertiary mesenteries has appeared wittiin the right middle exoccele (</* tig. 43, PI. V). 



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

 the right middle pair has lagged Ijohind. In ^-1. soltturla, 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. 



Clearl}-, 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 i^rimary systems, and within the 

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



Such has already taken place in fig. ^g in connection with the rigiit middle system, but a 

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



Polyps of AsfraiKjlaxiiUtdria 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, be obtained from the larger polyps of the closely allied 

 Phylldngia ainericana. 



