480 MEMOIRS OF THE NATIONAL ACIADEMY OF SCIENCES. 



Ill most instances, and especially in the perforate corals, the skeletotrophic layers comprise 

 the greater proportion of the soft parts of the colony upon decalcitication. The superficial 

 tissues as a whole — colmnn wall, tentacles, and disk — are always much less in superficial area 

 than the skeleton-covering tissues. The polyps, as a rule, extend a little more deeply within the 

 skeleton than is the amount of their expansion above. 



To study with any degree of success the skeletotrophic tissues of a coral, it is necessary that 

 decalcitication be carried out. Lining the corallum so very closely, it is impossible to make 

 a minute examination of the polypal layers in xitu with the thinness to which sections of the 

 skeleton can be ground. In the process of dissolving away the skeleton by means of acids, scarcely 

 any distortion of the tissues appears to take place if the latter have been properly hui'dened. and 

 the same may be said of the histology of the skeletogenic layer. 



The polypal region set free by decalcification is very complicated in its drtailed characters, 

 and varies greatly for each genus. An exact representation may be obtained by making a plaster 

 cast of the surface of auy dried coral, and then dissolving away the latter. The superficial tissues 

 seen in the living or preserved colony are found to represent but a small proportion of the polyp. 

 The space formerly occupied l)y the thecal wall is now free, and in the case of genuniferous species 

 each polyp presents much more individuality than under ordinary conditions, while in fissiparous 

 genera, like 2fxandrina and ColjJophylUa. the polypal systems stand out as very distinct, continuous 

 ridges, separated laterally by deep grooves formerly occuj)ied by the collines. The septal and 

 columellar projections are now represented by so many deep lateral and vertical inturnings of 

 the polypal tissues, and their arrangement can lie studied in detail, though adding little to what 

 is obtainable from the skeleton itself. 



The height of the decalcified polyp gives the depth to which in the living condition the 

 soft tissues extend downward within the corallum, revealing how comparatively superficial in 

 every case is the living portion of a colony. For example, after decalcification the polypal tissues 

 in Porites astra^oidt-s vary from 3 to 5 mm. in thick^iess; the polyps of Sidcrastnea railimin are 

 3 mm. in height when freed from the skeleton, and those of the larger 8. siderea are •! nun. 

 Polyps of OrlnceJJa (icrojxmi scarcely extend for 1 cm. within the skeleton of the colony, and the 

 same is the case even with the polypal sj'stems of the large colonies of Mixandrhui. 



In the upper region of decalcified polyps the skeletotrophic walls as a rule present a 

 diflercnt structural ai)pearance from those below. Al)ove, they are more transparent and 

 delicate looking, but as the lower region is approached the walls gradually become firmer in 

 character, white, and strongly opaque. This structural alteration is seen in nearly all the 

 forms examined, and is evidently due to the pronounced histological change, referred to l)elow. 

 which takes jjlace in the cndoderm in passing from al)0ve downward. 



Histologically the three skeletotrophic layers differ much among themselves, and also in 

 different regions of the polyp. They will now be described as seen in sections. 



ENDODERM. 



In the upper part of any polyp the skeletal endoderm. as a rule, closely resembles tliat of 

 the column wall, disk, and mesenterial epithelium; or, as in PI. XVIII, fig. 129, it maybe some- 

 what narrower, being represented by a very simple columnar epithelium. Gland cells of various 

 kinds, supporting cells, and scattered zooxanthelhv are the usual constituents, but no trace of 

 any muscular fibrils nor of a nerve la^'er has l)een found. 



As th(^ more proximal regions of the polyps are approached, the layer begins to undergo 

 a peculiar modification. It be(!omes much broader and is highly vacuolated, exhibiting in 

 sections a delicately reticular structure, the individuality of the cells being wholly lost (figs. 

 129, 73), while the chief constituents — nuclei, cytoplasm, zooxanthellre, and the contents of the 

 few granular gland cells — are mostly accumulated in a marginal zone. The differences in char- 

 acter l)et\veen the endoderm in th(> upper regions and l)elow are represented by figs. 44, 45, and 

 73. 7.'>. In OrhiceUd (PI. X. tig. 73) the layer is 0.1 mm. broad below, while above it is about 

 0.03 mm. in thickness (PI. IX. fig. (is). 



