MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 561 



at its lower termination it also thickens, then immediately thins out as it pusses into the 

 skeletotrophic tissues. The endoderm is a narrow, somewhat uniform layer, the cells of which 

 contain numerous zooxanthellse; clear gland cells are scarcely represented. 



In all the tentacles a battery of nematocysts occupies the apex, and smaller batteries occur in 

 different areas along the whole length of the stem. The former gives rise to the white apex 

 noticed among the external characters, and the latter correspond with tin' smaller elevations 

 alone- the stem of the tentacles. In sections of extended tentacles the ectoderm is swollen in 

 the region of the batteries, and narrow in the intervening spaces (fig. ."iii). The nematocysts of 

 the batteries are long and narrow, the internal spiral thread being so tine as to he determined 

 with difficulty. Two kinds occur, one about half as long as the other: they are practically 

 limited to a peripheral /one in the ectodermal layer, and this they crowd to the exclusion of 

 nearly all other elements. 



In the living condition the cnidocils are seen very distinctly as delicate triangular processes 

 from the surface of the ectoderm, and cilia occupy the areas between. The larger nematocysts 

 when shot out present an elongated oval cyst, and a long thread strongly barbed and thicker 

 proximally, hut very tine and smooth distally. Of the smaller nematocysts the thread is simple, 

 and only partly extruded in most cases. Elongated, deeply-staining, homogeneous bodies are to 

 he seen in the deeper parts of the ectoderm, and are no doubt developing nematocysts. Their 

 internal end borders almost directly on the mesogloea, so that in attaining maturity they 

 must migrate toward the periphery. 



The rounded nuclei of the ectodermal cells stain deeply, and stand out very distinctly from 

 the other parts of the layer. In sections they are arranged just within the nematocyst zone, and 

 a (dear zone intervenes between the musculature and the nuclear region. Large, ovoid, gland cells, 

 with coarsely granular contents, occur throughout the ectoderm, and occupy the greater proportion 

 <>f the layer in sections, but are not so numerous in the region of the batteries. The ectodermal 

 longitudinal musculature, though weak, is clearly distinguishable throughout the tentacles, the 

 mesogloea supporting it being slightly sinuous in some examples. Compared with its condition 

 in most other regions of the polyp the mesogloea of the tentacles is well developed. The 

 internal limitations of the endoderm are irregular, some, portions extending more within the 

 lumen than others; zooxanthellse are numerous, and the endodermal musculature is very weak. 



The ectoderm of the disk contains numbers of clear glandular cells, and an occasional 

 nematocyst; the endoderm is a broad layer, and zooxanthelhe are abundant within its cells. A 

 circular endodermal musculature can be readily distinguished, but no radiating ectodermal 

 muscle tiliers have been detected. 



In retracted polyps the central part of the disk is drawn somewhat internally, so that the 

 actual lips are formed by it. and here the endodermal musculature is most obvious; but a little 

 within the apparent mouth the ectoderm undergoes great histological modification, and assumes 

 the usual characters of the stomodieal epithelium. The stomodieum is oval in transverse sections, 

 so that the median axis of the polyp is easily determined. The walls are very short in vertical 

 sections, and folded vertically and transversely (tig. 56), and in some polyps the ridges opposite 

 the insertion of the mesenteries are well developed. At its lower termination the stomoda-al 

 ectoderm is strongly reflected, and passes for some distance along the two faces of the complete 

 mesenteries, becoming continuous with the mesenterial filaments (tigs. .">f. 60). 



The structure of the stomodseal wall is the same all the way round in any transverse section, 

 there being no histological differentiation distinguishing the ridges and furrows. Its ectodermal 

 layer begins to narrow below, and mesenterially is in continuity with the tissue of the filaments. 

 The ectoderm consists mostly of ciliated supporting cells, but long unicellular granular gland 

 cells, the contents of which stain deeply, are also present. Many of the narrow gland cells 

 extend outwardly as far as the margin of the layer, that is, a little beyond the nuclear zone, and 

 are thus very distinct for this part of their length (fig. 525). Long narrow nematocysts. and also 

 a large oval form, occur in the lower regions, and a great number of granular cells, especially 

 toward the mesoglceal limits of the layer. No trace of an ectodermal musculature or nerve 

 layer can be seen. 



