INIEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 465 



other; second, another pair appears within each of the remaining exocojiic chambers, thedilferent 

 members of the series of six paii's following the same succession as the first series of six pairs. 

 The regularit}' is liy no means strictly adhered to; growth in one sextant of the polyp may 

 be in advance of gi'owth 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 dispositioh 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 



Fig. 9i. 



' Phyllangia americana. — All the secondary mosenteries are now united with the stomodseum. and along with the members of the tirst order 

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

 appeared on the dorsal side. 



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

 stomodieum; 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 menii)ers of any one cycle, and in 

 the main cliaracteristic of the species. The adult arrangement lias 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 Mivandrtua and Colpoj^hylUa 

 (p. 440), it was found tluit the mesenteries at most are divisible into only complete and incomplete 

 pairs, but that the alternation is b\- no means constant. Sometimes several complete pairs are 

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



