22S 



CO-LENTERATA 



of ectoderm and penetrated liy a network of entodermal canals. On 

 ilisturbance the polyps retract into the civnosarc. 



Colonial Anthozoa, with few cxceplions, ha\'e a skeleton (corci!), 

 secreted bv the ectoderm, consislim; of calcic carbonate or of an cganic 



Fig. 196. — -\rrangement of septa in various .\ctin07.0a. T, Tctracoralla; II. Ccriaii- 

 Ihus. Ill, C)L-tocoralla. I\', Eji.'ardsij. 



horny substance, the two sometimes occurring together. The skeleton may 

 be internal (axial) secreted l)y the ccenosarc, or external (cortical), and 

 formed by the polyps, repeating to a large extent their complicated struc- 

 ture (figs, igg, joo). InFinigij (mushroom corals) the cortical skeleton 



Fig. 107- — i'l'i-allium niUnitn, reil corai (after Lac.i -.e IHilhiersV .1, ciliatcil 

 youiiLi; F\ )-ouni^ colony: C'. ]>ar( of co1on\' wilh pot\-j>s in extension ^1/) anil eontraetion 

 (1); (/, ceenosare; .1, greatly, li, t', sliglilly enlarged. 



consists only of a base, wilh radiating ridges (sclerose f>la) on the side to- 

 wards the llesh. These alternate wilh the sepia (sariosepla) of the polyp. 

 In most forms there is, in adtlilion, a cup (llicca) in the column of the 

 polyp, the sclerosepla extending inward from tliis. 



