IV 



PHYLUM CCELENTERATA 



121 



surface of the theca a number of radiating partitions, the 

 septa, proceed inwards or towards the axis of the cup, 

 some of them meeting in the middle to form an irregular 

 central mass or columella, which in some kinds of corals 

 forms an independent, pillar-like 

 structure arising from the middle 

 of the base. 



In the living condition the 

 polype fills the whole interior of 

 the corallite, and projects beyond 

 its edge to a greater or less degree 

 according to its state of expan- 

 sion. The septa alternate with 

 the mesenteries, each being in- 

 vested by an in-turned portion of 

 the body-wall ; so that, though 

 having at first sight the appear- 

 ance of being internal structures, 

 they are really external, lying alto- 

 gether outside the enteric cavity, 

 and are in contact throughout 

 with the ectoderm. 



The almost infinite variety in 



form of the compound corals is FIG. 60. Pennatula sulcata. 



. . Entire colony. Natural size. 



due, in the main, tO the Various /, lateral branch. (After 



methods of budding. According 



to the mode of budding, massive corals are produced in 

 which the corallites are in close contact with one another, 

 as in Astr&a (Fig. 62) ; or tree-like forms, such as Den- 

 drophyllia (Fig. 63, A), in which a common calcareous 

 stem, the ccznenchyma, is formed by calcification of the 

 ccenosarc, 1 and gives origin to the individual corallites. 

 1 See p. 94. 



