204 



ZOOLOGY 



SECT. 



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 

 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 



FIG. 157. Dendrophyllia nigrescans, li, XKTadrepora aspera. 



co. corallites ; cs. coenosarc ; p. polypes. (After Dana.) 



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

 sponds with a radial in-pushing of the base, and is formed as a 

 secretion of the invaginated ectoderm. As the septa grow they 

 unite with one another at their outer ends, and thus form the theca. 

 In some cases, however, the theca appears to be an independent 

 structure. 



The almost infinite variety in form of the compound corals is 

 due, in the main, to the various methods of budding, a subject 

 which has already been referred to in treating of the actinozoan 

 colony as a whole. According to the mode of budding, massive 

 Corals are produced in which the corallites are in close contact 

 with one another, as in Astraea (Fig. 147) ; or tree-like forms, such 



