ADULT COLONY. 33 



stinging cells; and further, the cysts are usually present in numbers at 

 the aboral extremity of the larva which becomes the basal disc when the 

 larva settles. So far as can be made out from sections, without isolation by 

 maceration, the nematocysts in the calicoblast layer are similar to those in 

 the ectoderm of the column wall. Bourne (1899), in his studies of the 

 calicoblast layer of the Madreporaria, also encountered oval bodies which 

 suggested to him degenerate nematocysts. The close spiral thread found 

 in those of Siderastrea, and their general behavior at different stages towards 

 reagents, leave no doubt as to their true nature. 



The structure of the calicoblast ectoderm, as above given, differs some- 

 what from that usually met with in corals. As described in the papers of 

 Bourne, Fowler, and Gardiner, the layer is thickest in the regions of active 

 growth, and becomes very thin or nearly disappears elsewhere. Such is also 

 its condition in most of the species of corals examined by me (1902, p. 482). 

 It may appear either as a columnar epithelium (Madrepora) or have the 

 character of a syncytium. A well-developed cellular tissue, with abundant 

 protoplasm, would be expected where active formation of the skeleton is in 

 progress. Here, in Siderastrea, the layer is thickest in those regions where 

 the secretory activity would be considered to be at a minimum. The granular 

 character of the cytoplasm, the small size of its nuclei, and the strong 

 vacuolization would not, however, suggest that the deposition of skeletal 

 matter was actively proceeding in the lower regions, as compared with the 

 large nuclei and more hyaline cytoplasm along the edges of the septa. 



It seems doubtful as to how far the thinness of the endoderm in the upper 

 region is to be associated with a similar condition of the calicoblast endoderm. 

 Over the spines of the colurnella, which are situated in the lower regions of 

 the polypal cavity, and are probably in a growing state, the two layers pre- 

 sent a great difference in this respect. The ectoderm is as thin as in the 

 uppermost growing parts of the polyp, while the endoderm is greatly thick- 

 ened, measuring as much as 0.05 mm. across, which is nearly equal to the 

 combined thicknesses of both ectoderm and endoderm in the lower regions. 



None of the writers on the anatomy of coral polyps have referred to the 

 marked alteration undergone by the skeletotrophic endoderm in passing from 

 the upper to the lower parts of the polyp. It is, however, characteristic of 

 practically all West Indian corals, the contrast being usually greater than 

 that shown by Siderastrea. 



In his study of the minute anatomy of the polyps of Flabellum rubrum, 

 Gardiner (1902) found the calicoblastic ectoderm to be in many respects 

 like that of S. radians. The layer is everywhere complete and persistent, 



