5(38 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



irritation, and are again withdrawn. New individuals arise amongst the others at any spot along 

 the line of union of contiguous polyps, and also around the margin. 



Anatt imi and histology. — The column wall is very thin in microscopic sections, the mesogloea 

 being scarcely determinable as a distinct layer. The ectoderm is constituted almost wholly of 

 clear gland cells, and in vertical sections these are regularly arranged and lie closely together; 

 nuclei occur in the intervals between one gland cell and another, and give rise to a discontinuous 

 zone about the middle of the layer. In the more internal parts occur smaller gland cells with 

 granular contents, and also a number of nuclei closely apposed to the mesogloea. The endoderm 

 is very thin, often not more than the diameter of a single zooxanthelhv across. No muscle 

 fibrils are seen on either side of the mesogloea, not even in the capitular region of retracted 

 polyps (figs. 76-78). 



The tentacular ectoderm is a broad layer, especially at the tip. where it is constituted largely 

 of supporting and nematocyst- bearing cells; fewer clear gland cells, and more of the granular 

 variety, occur than in the column wall. In retracted specimens the lateral nematocyst batteries 

 stand out prominently, while the intervening areas are very narrow (rig. 75). The nematocysts 

 me mostly of the, long, narrow, thin-walled form, and an occasional large oval specimen may be 

 present, especially at the tip. A distinct longitudinal ectodermal muscle layer occurs, while the 

 endodermal musculature is weak, becoming a little stronger proximally. The endoderm is 

 richly supplied with zooxanthellse. 



The disk presents no important histological differences from the column wall. The stomoda-um 

 is very short in fully retracted polyps, its walls being greatly folded transversely, but in the 

 examples studied the ectoderm presents no special vertical folds with mesogheal thickening, 

 although strong ridges are noted among the external characters. 



The mesenteries are in two cycles of six pairs each, the first only reaching the stombdssum 

 (rig. SI). Two pairs of directives occur in all cases. Beyond the primary and secondary cycles 

 new mesenteries are added in double unilateral pairs, of which one pair becomes complete and 

 the other remains incomplete, in such a manner that no additional third cycle is formed. Such 

 enlarged polyps ultimately undergo fissiparous gemmation. A septal invagination of the skeleto- 

 trophic tissue occurs within each entocoele and exoccele. 



The mesenterial endoderm bears numerous clear mucous cells and zooxanthellse. The 

 mesogloea is a broad, (dear, homogeneous layer above, but narrows below. For sonic distance. 

 from the line of origin of the mesentery one face of the mesogloea is thrown into numerous close 

 folds for giving increased support to the retractor muscle, but the opposite face remains smooth. 



Mesenterial filaments occur on all the mesenteries, and are somewhat exceptional both in form 

 and structure. The filament is nowhere sharply distinguished from the mesenterial epithelium, 

 but the two tissues pass insensibly into one another. The free end of the mesentery is simply 

 clavate, the thickened region, whether filament or mesenterial endoderm, consisting mostly of 

 ciliated supporting cells (tig. 85). At the actual tip one or two nematocysts may occur, and in the 

 lower convoluted region numerous large oval nematocysts are found as usual. With such a form 

 and structure the mesenterial filaments bear very (dose comparison with the incipient stages of 

 the filaments on the incomplete mesenteries of most other corals. 



The skeletotrophic endoderm is narrow in the upper region of the polyp, but becomes very 

 broad below (rigs. 75, 79). Zooxanthelbe are present throughout, and in the lower region gland 

 cells with exceptionally large granules begin to be numerous; the individual granules are nearly 

 as large as the zooxanthellaa and are highly refractive. As the lower regions are approached the 

 endoderm undergoes an increase in thickness; the zooxanthellse and granular gland cells are still 

 plentiful, and along with the nuclei are nearly all aggregated toward the periphery of the layer. 



The supporting lamella of the skeletotrophic tissues is very thin throughout, only clearly 

 determinable as a distinct layer at the place of origin of the mesenteries, and in these regions 

 desmoidal processes can be distinguished. The skeletogenic ectoderm remains as a somewhat 

 distinct layer, even in the most proximal regions, though in some places more than others 



•' Fissiparous gemmation has since been found i :cur. 



