MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 587 



calice, so thai transverse sections show an outer and an inner columnar wall before ili«' tentacular 

 zone is reached. The inner chambers arc bere 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 

 asrsrresrated 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 mesoglcea and 

 endoderin are extremely thin, and the latter contains zooxanthellse. Nematocysts are apparently 

 absent from the ectoderm, and only 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 mesogloeal processes are produced for attachment alone- the skeletal grooves, but are 

 weak in character, and continue to he observable for some distance away from the mesenteries; 

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

 distribution 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 reti-acted 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 mesoglcea, and from 

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

 and also give rise to a weak endodermal musculature. 



The stomod.vum 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 with the attachment of the 

 mesenteries. Owing to the obliquity of the polyps, one end of the stomodseum generally termi- 

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

 little distance along the two fares of each of the complete mesenteries, and the mesenterial fila- 

 ments of all the perfect mesenteries appear as if continuations of the stomodseal ectoderm. The 

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

 gland cells, with fine granular contents; in contrast with the gland cells of the column wall, 

 these stain deeply and extend beyond the nuclear zone to the free surface of the ectoderm. A. 

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

 mesoglcea is extremely thin, and no musculature is determinable on either side of it; the 

 endoderin contains many zooxanthellse. 



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

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

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

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

 is possible to make out the two pairs of directives. 



The mesenterial filaments on the complete mesenteries are in continuity with the stomodseal 

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

 cells, eland cells, and narrow nematocysts. At first the filaments are cordate in section: later 

 they arc nearly circular, and the mesenterial epithelium behind is swollen, so that a trilobed 

 character is given to the free extremity of the mesentery as a whole. In the lower regions the 

 mesenteries become convoluted, and the filament is not sharply marked off from the endodermal 



epithelium. Large and small nematocysts. similar to those in the ectoderm of the sti idaeum, 



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

 teric-, which never reach the stomodseum, are first indicated in the distal region by a small 

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

 exactly resemble those of the complete mesenteries, and in the lower region it is impossible to 

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

 tain- numerous zooxanthellse. 



