iv PHYLUM CCELENTERATA 123 



has the form of a short conical cup, much compressed, so 

 as to be oval in section. Its wall or theca is formed of 

 dense stony calcium carbonate, the proximal end produced 

 into a short stalk or peduncle. From the inner surface of 

 the theca a number of radiating partitions, the septa, pro- 

 ceed 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 mdepend- 



FIG. 59. Astraea pallida. the living colony. (After Dana.) 



ent 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 expansion. The septa 

 alternate with the mesenteries, each being invested by an 

 in -turned portion of the body -wall ; so that, though having 

 at first sight the appearance of being internal structures, 

 they are really external, lying altogether outside the enteric 

 cavity, and are in contact throughout with the ectoderm. 



