104 GEORGE MATTHAI 



granular appearance being more pronounced than elsewhere 

 in the ectoderm. Nuclei are few and are arranged in a single 

 row ; they are large, oval or round, rarely elongated, finely 

 granular, and lie tangentially in a single row at comparatively 

 wide intervals, except near the attachments of mesenteries 

 to the corallum, where the calicoblastic layer is usually thickened 

 and nuclei tend to Ijecome irregularly distrilnited without any 

 crowding. Most of the nuclei contain a Inilliantly stained spot 

 (perhaps the nucleolus) which is conspicuous under an oil- 

 immersion lens. The calicoblastic layer persists to the base 

 of polyps, though considerably attenuated. 



From this account it will be seen that the ectoderm 

 consists of two histologically different regions, viz. the part 

 covering the exposed surface of colonies (i.e. the oral-disc, 

 edge-zone, and coenosarc ectoderm) and the calicoblastic 

 layer, the differentiation being doubtless in accordance with 

 differences in their functions. In the free-swimming larva of 

 Favia fragum, the ectoderm of the column- w^all (fig. 4) is 

 histologically similar to that of the oral- disc of the polyp 

 (the calicoblastic layer being non-existent at this stage), 

 and is, in this respect, comparable to the column-w^all of 

 Actinians ; at the aboral pole the ectoderm is thickened, 

 presumably for future attachment. Tentacles had not appeared 

 in any of the larvae examined. 



Endoderm. 



The endoderm forms the lining of the coelenteric cavities, 

 i.e. forms the inner lining of the column-wall, oral-disc, edge- 

 zone, and tentacles, the outer lining ot the stomodaeum and the 

 double sheet of the mesenteries. It is usually vacuolated and is 

 apparently without cilia, while in Actinians the Hertwigs 

 found a single long cilium or flagellum on each endoderm cell ; 

 the vacuoles are often elongated, their longer axes are more or 

 less perpendicular to the width of the mesenteries, the vacuoles 

 being somewhat broader distally. Nuclei are oval or round, 

 smaller and less numerous than those of the oral-disc ectoderm. 

 The endoderm varies in thickness in different parts of the same 



