MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 465 



other; second, another pair appears within each of the remaining exoccelic chambers, the different 

 members of the scries of six pairs following the same succession as the first scries of six pairs. 

 The regularity is by no means strictly adhered to; growth in one sextant of the polyp may 

 lie in advance of growth in another, independently of the general dorso-ventral succession. 

 Part or all of the twelve pairs necessary to complete the order may be characteristic of any 

 species. Ultimately all the tertiary pairs attain the same radial extent, which is less than that 

 of the secondaries. 



APPEARANCE OF MESENTERIES IN POLYPS REPRODUCING BY FISSION. 



All the examples referred to above, as attaining a cyclical disposition of the mesenteries in 

 the adult polyp, are species reproducing asexually by gemmation. A perfect regularity, as regards 

 the radial length of the mesenteries of the different cycles, obtains in these, exactly as in sexually 



Phyllanffia ami ricana.— All the secondary mesenteries are now united with the stoinodseum, and along with the members of the first order 

 (protoenemes) constitute the first cycle of mesenteries. Four pairs of third-cycle metacnemes (fourth-order mesenteries, IV) have 

 appeared on the dorsal side. 



produced polyps. The organs do not continue their growth indefinitely until reaching the 

 stomodseum; only the members of the first order of six pairs, or, in larger polyps, those of 

 the second order also, become united with the stomodseum. The remaining orders extend for 

 definite radial distances from the body wall, uniform for the members of any one cycle, and in 

 the main characteristic of the species. The adult arrangement has been shown to be otherwise 

 with species in which asexual reproduction by oral fission prevails; and this whether the new 

 polyps become distinct, each with its own tentacular system, or whether they remain incompletely 

 separated, and give rise to meandering tentacular and discal systems (p. 448). 



In describing the mesenterial arrangement in the genera Mseandrina and Colpqphyttia 

 (p. 449), it was found that the mesenteries at most are divisible into only complete and incomplete 

 pairs, but that the alternation is by no means constant. Sometimes several complete pairs are 

 found without any intervening incomplete pairs, while, when the latter do occur, they are very 



