206 



ZOOLOGY 



SECT. 



vested by an in-turned portion of the body-wall (E, F). Thus the 

 septa, which appear at first sight to be internal structures, are 

 really external : they lie altogether outside the enteric cavity, and 

 are in contact throughout with ectoderm. 



The ectodermal nature of the entire corallite is further proved by 

 its development. The first part to appear is a ring-shaped 



GCfi.ff 



Fio. 155. A, B, two views of Flabellum curvatum. C, semi-diagrammatic view of a simple 

 coral ; D, portion of a corallite ; E, F, diagram of a simple coral in longitudinal and transverse 

 section ; ectoderm dotted, endoderm striated, skeleton black, b. pi. basal plate ; col. colurn- 

 ella ; e. th. epitheca ; yul. gullet ; mes, mes. 1, mes. 2, mesenteries ; mes. f. mesenteric filaments ; 

 sep. septa ; t. tentacle ; th. theca. (A and B after Moseley ; C and D after Gilbert Bourne.) 



deposit of carbonate of lime between the base of the polype and the 

 body to which it adheres : sections show this ring to be formed by 

 the ectoderm cells of the base. The ring is soon converted into a 

 disc, the basal plate, from the upper surfaces of which a number of 

 ridges arise, arrayed in a star-like fashion : these are the rudiments 

 of the septa Here, again, sections show that each septum corre- 



