512 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Fig. It',/ represents the mesenterial plan in the second bioral polyp. In the living condition 

 one oral aperture was much smaller than the other, appearing as a mere perforation in the disk, 

 and sections reveal that a less number of complete mesenteries are associated with it than with 

 the larger. In the diagram the smaller stomodseum is to the left, but is represented equal 

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

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

 stomodseum 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 toward the dorsal surface. 



Attention may now be directed to the fully developed polyps constituting a colony of Favia, 

 m order to ascertain what are the results of fission upon these. As already remarked, the 

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

 They are rarely circular in contour, but polygonal or greatly elongated, and at times deeply 

 angular; in the majority of adult polyps only one oral aperture is surrounded by a tentacular 

 system, 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, differing greatly from 



Fir;. 111;. 

 Faviafragum. — First stage of fission in another larval polyp. 



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

 no regular hexameral cyclic arrangement can be established. Different stages of growth are 

 represented in different 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 together. 



In the upper part of the stomodseum 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 they are not all of the same ordinal value. 



The mesenterial pairs are always isocnemic. and the retractor muscles are invariably on the 

 faces turned toward one another; in transverse sections of over a dozen polyps examined no 

 directives occurred. 



Adult polyps of the genera Tsophyllia (p. 449), Agaricia (fig. 161), Mamndrina (tig. Ill), 

 Colpop7iyllia, and Dichoccenia (rig. 119) display a like irregularity of mesenterial arrangement 

 and absence of directives. The actual stages in fission 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 Manicma and Favia. 



Several Actinias also exhibit the phenomenon of fissiparity, and certain investigations have 

 been made as to its influence upon the mesenteries and other organs. Dr. G. H. Parker (1899) has 



