POSTLARVAL DEVELOPMENT. 123 



endoderm also has undergone a great alteration compared with the same 

 layer in the other regions of the polyp. It is much thinner, all traces of cell 

 limitations are lost, and Zooxanthellse are wholly absent, while elsewhere in 

 the endoderm the algal cells occur in some abundance. 



The passage from the calicoblastic ectoderm to the ectoderm of the 

 column wall is gradual, and can be studied on both sides of the section. It 

 is at this point that the epitheca is laid down. Were there any doubt as to 

 the epithecal or thecal nature of the peripheral calcareous ring such sections 

 prove absolutely that the deposit is uncovered by the polypal tissues on its 

 outer sides. Were it otherwise an invagination of the base would occur 

 towards the periphery, but none of the sections show any such folding. 



At the region of the epitheca there is no sharp distinction between the 

 column wall and the basal disc. The ectoderm of the former is a broad 

 columnar epithelium, having many clear unicellular mucous glands distrib- 

 uted among the supporting cells ; the nuclei are restricted mostly to the inner 

 half of the layer. The left half of the disc includes the section of a tentacle, 

 represented only by the knob which is crowded with long, narrow nematocysts. 

 As in adult retracted polyps, the stem of the tentacle is not distinct from 

 the disc. Histologically the walls of the stomodseal invagination closely 

 resemble those of the adult polyps, and the ectoderm is marked off from the 

 rest of the body wall by its numerous ciliated supporting cells. 



On both sides of the section represented in fig. 53 the stomodaeal wall 

 terminates freely, but in sections which include a mesentery the ectoderm is 

 seen to be continuous with the mesenterial filaments along the free edge of 

 the mesentery. The two, stomodasal ectoderm and mesenterial filament, are 

 much alike histologically, and by their brightly-staining character are easily 

 recognizable among the other tissues. 



