512 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Fig. 16/ represents tlie mesenterial plan in the second hioral polyp. In the living- condition 

 one oral aperture was nuicii smaller than the other. api)earing as a mere perforation in the disk, 

 and sections reveal that a less number of complete mes(Miteries are associated with it than with 

 the lai'ger. In the diagram the smaller stomodasum is to the left, but is represented (njual 

 with the other. In the actnal transverse sections it displays eight strongly marked vertical 

 ridges, corresponding with the eight mesenteries attached to its inner side, while the large 

 stomodEemu bears ten. Here, again, it is seen that the plane of fission passes through the 

 entocoele of two opposite pairs of lateral mesenteries, and growth is proceeding more rapidly at 

 one region — to the lower right — of the polyp than at another, so that the directive axis does 

 not coincide with the longer diameter, but is turned towaid the dorsal surface. 



Attention may now lie directed to the fully develop(>d polyps constituting a colony of Favia, 

 ni order to ascertain what are the results of tission ujion these. As already remarked, the 

 mature polyps are found to exhibit very varied conditions with regard to the stage of tission. 

 They are i-arely circular in <-ontour, l>ut poh-gonal or gi-eatly elongated, and at times deeply 

 angular; in the majority of adult polyps only one oral apertui'e is surrounded ])y a tentacular 

 S3'stem, but sometimes two or three mouths occur on a single disk. 



A transverse section of a decalcified polyp is represented on PI. XIII. fig. 93, and indicates 

 much variability and irregularity in the disposition of the mesenteries, ditlering greatly from 



Klfi. Hit. 

 Far/It friifimu. — First stage of fission in nnnthi-r larv:il polyp. 



the perfect regularity of the early larval polyps. The organs are paired throughout, but 

 no reo-ular hexamei'al cyclic arrangement can be established. Difl'ei-ent stages of growth are 

 represented in dift'erent regions; in some places there is an indication of a tricyclic plan, but 

 more often only a dicyclic arrangement is manifest, and at times this is obscured by three or 

 four pairs of mesenteries of equal ordinal value occurring togethei-. 



In the upper jiart of the stomodteum all the mesenteries may be complete, except a pair here 

 and there in process of growth, but in passing downward some pairs become free in advance of 

 others, indicating that tliey are not all of the same ordintd vtilue. 



The mesenterial pairs are always isocnemic, and the retrat-tor nuiscles are invariably on the 

 faces turned toward one another; in tran.sverse sections of over a dozen polyps examined no 

 directives occurred. 



Adult polyps of the genera iKOpliyllia (p. 4iit), Ayaririd (tig. Itil), Ilsfandriita (tig. lil), 

 ColpophyUif-U and Diclioccania (fig. 119) display a like irregularity of mesenterial arrangement 

 and absence of directives. The actual stages in tission have not been traced in these, but from 

 their prevailing mesenterial arrangement it is manifest that the process proceeds in the same 

 way as in the young polyps of Manicina and Favia. 



Several Actinite :dso exhil)it the phenomenon of fissipai-ity. and certain investigations have 

 been made as to its influence upon the meseutei'ies and other oi'gans. Dr. G. H. Parker (1S99) has 



