576 MEMOIRS OF THE NATIONAL ACADEMY OF .SCIENCES. 



proximal region, but occurs more distally, and an ectodermal musculature is strongly developed. 

 The endoderm contains few zooxanthellre. 



The peripheral ectoderm of the disk presents a wide contrast from that of the more central 

 ana. The latter very closely resembles the outer layer of the column wall, being opaque 

 throughout, with the granular matter strongly developed; at the periphery, however, the 

 ectodermal cells are longer, a larger number of clear gland cells are present, and little granular 

 matter occurs. A few nematocysts may also he present, but apparently no ectodermal musculature 

 is developed. The mesoglcea is a little thinner than in the column wall, and its endodermal aspect 

 is plaited for the support of the musculature, most marked in the peripheral region. 



Transverse sections of the stomodanim display strong vertical ridges opposite the insertion 

 of the mesenteries, while the intervening areas are much thinner; large oval nematocysts occur 

 in the ectoderm of the ridges, but arc absent from the grooves, which in their turn are more 

 strongly ciliated. The ridges and furrows thus present somewhat the same histological differ- 

 ences which exist between the general ectoderm and the gonidial grooves in Actinians. The 

 granular pigmentation characteristic of the ectoderm of the column wall is absent from the 

 stomoda^al ectoderm: the mesoghea is everywhere thin, and only a weak endodermal musculature 

 is developed. 



The mesenteries are without any regular C3 r clic arrangement, and no directives occur. The 

 greater number of the pairs are complete, but alternating incomplete pairs are also present in 

 places, the different pairs varying in size. The incomplete members arc evidently recently 

 developed pairs which in time will become complete. 



The mesoghea on the mesenterial face bearing the retractor muscles is wavy in transverse 

 sections, or forms numerous plaits, which, however, vary greatly in the extent of their develop- 

 ment, both in different mesenteries and in different regions of the same mesentery. In the 

 middle region of some, of the mesenteries both mesoglceal faces are sinuous for some distance, and 

 the oblique musculature on the opposite face is strongly developed. The mesenterial epithelium 

 is narrow above, but becomes very broad below, and consists mainly of clear gland cells; 

 zooxanthelhe are also present. In the lower region most of the mesenteries become much 

 convoluted, and nearly till the septal loculi. Mesenterial filaments are borne on all the mesenteries. 

 whether complete or incomplete; the mesenterial endoderm behind is swollen in some instances 

 and not in others. Clear gland cells are somewhat numerous in the upper course, and in the 

 lower are many large oval nematocysts. In some cases the filament has undergone complete 

 glandular modification, and the areas stand out very prominently in sections, as the contents of 

 the eland cells are a deep yellow. The, modification is limited to the filament, without involving 

 the endoderm behind. 



The skeletotrophic tissues are characterized by the great thickness of the endoderm in the 

 lower regions, and by its almost complete vacuolization. Actual cell outlines have altogether 

 disappeared, and the few protoplasmic contents are aggregated in a narrow marginal zone. The 

 calicoblast layer is broad in some regions; numerous desmoidal processes occur alone- the 

 course of the insertion of the mesenteries, and here the mesoghea is much broadened. 



The septal invaginations are only entoccelic, and do not encroach much upon the gastro- 

 ccelomic cavity until near the aboral end of the polyp, but even here the interseptal chambers 

 are not distinct from one another. 



Female gonads were present on some of the mesenteries, restricted in their distribution 

 toward the insertion of the mesentery in the body wall. Generally only three or four ova are 

 seen in a transverse section of any mesentery, and may occur on either the complete or 

 incomplete mesenteries. 



