PHYLUM CCELENTERATA 



207 



spends 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 Astra?a (Fig. 146) ; or tree-like forms, such 



FIG. 15(3. Dendrophyllia nigrescang, B, Madrepora aspera. co. corallites ; 

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



as Dendrophyllia (Fig. 156, A), in which a common calcareous stem, 

 the ccenenchynia,i$ formed by calcification of the coenosarc (cs.), and 

 gives origin to the individual corallites. It is by this last-named 

 method, the coenosarc attaining great dimensions and the indivi- 

 dual corallites being small and very numerous, that the most 

 complex of all Corals, the Madrepores (Madrepora, Fig. 156, B) 

 are produced. 



The microscopic structure of corals presents two main varieties. 

 In what are called the aporose or poreless corals, such as Flabellum, 

 Astraea, &c., the various parts of the corallite are solid and stony, 

 while in the perforate forms, such as Madrepora, all parts both of 



