MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 561 



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

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

 contain numerous zooxanthellre; 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 the smaller elevations 

 along 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 (tig. 50). The nematocysts of 

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

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

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

 nearly all other elements. 



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

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

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

 l^roximally, but very tine and .smooth distally. Of the .smaller nematocysts the thread is simple, 

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

 be 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 deeplj', and stand out very distinctly from 

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

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

 with coar.seh' granular contents, occur throughout the ectoderm, and occupy- the greater proportion 

 of the laj'er 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 

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

 in mo.st 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 fibers 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 aj^parent mouth the ectoderm undergoes great histological modification, and assumes 

 the usual characters of the stomoda?al epithelium. The stomodwum 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 verticall}' and transversely (fig. 56), and in some polyps the ridges opposite 

 the insertion of the mesenteries are well developed. At its lower termination the stoniodasal 

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

 mesenteries, becoming continuous with the mesenterial filaments (figs. 51, 60). 



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

 there being no histological difl'erentiation 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 granulai' gland 

 cells, the contents of which stain deeply, ai-e also present. Many of the narrow gland cells 

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

 are thus very distinct for this part of their length (fig. 5%). Long narrow neniatoc^-sts, and also 

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

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

 laj-er can be seen. 



