MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 587 



calice, so that transverse sections show an outer and an inner cohminar wall before the tentacular 

 zone is reached. The inner chambers are here partly subdivided by the septal intrusions. 



The ectoderm of the outer wall is made up almost entirely of unicellular mucous glands, 

 the contents of which are perfectly clear; supporting- cells surround each gland cell, their 

 aggregated nuclei giving rise to a distinct middle zone. Gland cells with granular contents occur 

 in the deeper parts of the layer, and small deeply-staining nuclei. Both the mesoglu-a and 

 endoderm are extremely thin, and the latter contains zooxanthelliv. Nematocysts are apparently 

 absent from the ectoderm, and oidy the weakest endodermal musculature can be detected in the 

 upper region. Where the mesenteries are united to the outer walls of the corallum the usual 

 striated mesoghjeal processes are pi'oduced for attachment along the skeletal grooves, but are 

 weak in character, and continue to be observable for some distance awav from the mesenteries; 

 in fact, they occur somewhat freely throughout the skeletal tissues. Perhaps the increased 

 distril)ution is in some way determined by the unusual smoothness of the corallum, rendering 

 increased attachment for the soft parts necessary. The skeletogenic ectoderm is extremely 

 narrow, even in the growing parts of the corallum. 



The tentacles are very short in retracted specimens; the ectoderm is deeply folded, and much 

 swollen at the stinging areas. The apex is the broadest part of the layer, and the nematocysts 

 there are of two kinds: a small, narrow, thin-walled form, which also occurs in the lateral areas, 

 and a large, oval, thick-walled form restricted to this region of the tentacle. The layer also 

 contains numerous clear and granular gland cells, similar to those in the column wall. An 

 ectodermal musculature is clearly distinguishable on slight sinuations of the mesogloea, and from 

 it delicate fibrils pass to a nerve layer. The endoderm cells contain numerous zooxanthellse, 

 and also give rise to a weak endodermal nuisculature. 



The stomoda?um is oval shaped in transverse sections, and the ectoderm is thrown into five or 

 six folds on each side, which, however, bear no constant relationship witli the attachment of the 

 mesenteries. Owing to the oblicjuity of the polyps, one end of the stomodicum generally termi- 

 nates in advance of the other in a series of transverse sections. The ectoderm passes for some 

 little distance along the two faces of each of the complete mesentei-jes, and the mesenterial fila- 

 ments of all the perfect mesenteries appear as if continuations of the stonioda'al ectoderm. The 

 stomodseal ectoderm is constituted of ciliated supporting cells, among which are long, narrow, 

 gland cells, with tine granular contents; in contrast with the gland cells of the cohunn wall, 

 these stain deeply and extend beyond the nuclear zone to the free sui'face of the ectoderm. A 

 few large nematocysts are also scattered about, but apparently none of the smaller forms. The 

 mesoglcjea is extremely thin, and no nuisculature is determinable on either side of it; the 

 endoderm contains many zooxanthellic. 



Six pairs of mesenteries reach the stomodajum, while other six alternating pairs remain 

 incomplete throughout. The musculature is extremely weak in the upper region, so that it is 

 (lithcult to distinguish whether directives are present or not. In the proximal regions the 

 musculature becomes better developed, and is supported on delicate mesogloeal folds, and here it 

 is possible to make out the two pairs of directives. 



The mesenterial tilaments on the complete mesenteries are in contiiuiity with the stomodasal 

 ectoderm, and histologically the two are nuich alike, being constituted of ciliated supporting 

 cells, gland cells, and narrow nematocysts. At first the filanu>nts are cordate in section; later 

 thev are nearly circular, and the mesenterial epithelium liehind is swollen, so that a trilobed 

 character is given to the free extremity of the mesentei'y as a whole. In the lower regions the 

 mesenteries become convt>luted, and the filament is not sharply marked ott from the endodermal 

 epithelium. Large and small iu"natocysts, .similar to those in the ectoderm of the stomoda'um, 

 are numerous in some of the filaments, but not in all. The filaments on the imperfect mesen- 

 tei-ies, which never reach the stomoda-um, are first indicated in the distal region ))y a small 

 group of deeply-staining nuclei at the free extremity; soon, however, they develop .so as to 

 exactl}' resemble those of the complete mesenteries, and in the lower region it is impossible to 

 distinguish between the tilaments of the two cycles. The mesenterial endoderm throughout con- 

 tains numerous zooxantlu'lLe. 



