MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 481 



In some species the thickened endodenii is crowded throughout its extent with granules 

 of various size, which render the layer dense and nearly opaque in sections. This is especially 

 characteristic of jUtrangia soli.fnria (PI. V^, figs, -i-i, 45) and /)/c/ioc(enia. sfokesi. The granular 

 particles are usually non-staining, and are thus distinguished from the nuclei which are also 

 present, situated near the margin. In Dlcliocmiia the granules are green (p. -i.^O). Very rarely 

 they are arranged as if constituents of an oval cell; more usuallj' they arc scattered uniformly 

 through the whole or part of the layer, without any suggestion of being contained in special 

 gland cells. The condition in the lower skeletal endodermof Solenastrxa, represented on PI. X, 

 fig. 79, is somewhat intermediate; large cells full of coarse granules occur, and in addition 

 to these are many isolated granules. 



The thickened skeletotrophit; endoderm of the rissiparous genera 2fxandi'i)Hi, 3Lnricina, 

 ColpophylUa, and also Orblcellu, has very few contents; a few small scattered nuclei, here and 

 there a zooxanthella, and perhaps a few granules, are all that can be made out, the layer being 

 vacuolated in either a rounded or irregular manner. In the porose genera Madrejxira and 

 Porltes practically no modification ocicurs; the skeletotrophic endoderm is much alike in character 

 in all parts of the polypal cavity, as well as in the canalicular outgrowths. Also in Stdenintrxa 

 .scarcely any ditierence is api^arent between the upper and lower skeletotrophic endoderm 

 (PI. XXIII, fig. 1.56). This genus is fui'ther exceptional in that the calicoblast ectoderm I'emains 

 a broad layer throughout. 



The great thickening of the endoderm sensibly diminishes the mesenterial loculi l)elow; 

 while the comparative fewness of the nuclei, their small size, and the .sparse protoplasmic 

 contents would indicate that the cellular activity is much diminished compared with the upper 

 regions of the polyp. 



Wherever the calicoblast layer is in an active condition the endoderm overlying it presents 

 a corresponding state. In the upper parts of polyps, where skeletal growth is proceeding as a 

 result of the activity of the calicoblasts, the endodermal cells overlying the latter are highly 

 protoplasmic, stain deepl}', and present all the evidence of functionally active cells. A marked 

 instance of this occurs in connection with the aboral termination of the interseptal loculi. It is 

 here that from time to time the dissepiments are formed which cut off the pol3'p from the lower 

 portion of the corallum; below the last dissepiment the skeleton may be considered as dead, 

 while above it is covered with the soft polypal tissues. The production of dissepiments must 

 be constantly taking place in a vigorously growing coral, hence the calicoblasts at the actual 

 base remain in a more or less permanent condition of activity, as represented on PI. X, fig. 73. 

 The figure shows that the colunmar character of the cells is limited to the actual fiat base of 

 the chambers, the calicoblasts l)eing insignificant along the lateral walls. Fig. 73 is also 

 specially instructive as showing how the endoderm immediately overlying the active calicoblast 

 laver differs from the layer on the lateral walls, where the calicoblasts are non-active. The 

 endoderm has Ijecome much thinner, the cells are fully charged with j)rotoplasmic contents, 

 and stain deeply. 



The skeletotrophic endoderm overlying the upper parts of septa which mav be supi)osed to 

 be in a growing condition, is also uuich thinner than that lining the wall of the calice and inner 

 parts of the septa at the same level. This diversity is very marked in fig. 129, Pi. XXIII, 

 representing a mesentery of Miv<tndrina with the skeletotrophic tissues associated with it. 



It is manifest that the outer calicoblasts can obtain their nutriment and the calcareous salts 

 wherewith to form the dissepiments only in so far as these pass through the overlying endoderm 

 and mcsoglcea; hence wherever the former are in a functionally activ^e condition the endoderm 

 would be expected to show a corresponding modification, as compared with regions where it 

 overlies non-functional cells. 



MESOGLCEA. 



The mesolgoea of the skeletotrophic tissues is nearly everywhere a thin lamella, but, as a 

 rule, it thickens a little along the line of attachment of the mesenteries to the corallum. At this 

 place, and more or less scattered over the whole surface, are found peculiar cone or wedge-shaped 



