228 



CCELENTERATA 



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

 disturbance the polyps retract into the coenosarc. 



Colonial Anthozoa, with few exceptions, have a skeleton (coral), 

 secreted by the ectoderm, consisting of calcic carbonate or of an organic 



FIG. 196. Arrangement of septa in various Actinozoa. I, Tetracoralla; II, Cerlan- 



thus; III, Octocoralla; IV, Edwardsia. 



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

 be internal (axial) secreted by the coenosarc, or external (cortical), and 

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

 ture (figs. 199, 200). \i\Fimgia (mushroom corals) the cortical skeleton 



FIG. 197. Corallium rubrum, red coral (after Lacaze Duthiers). A, ciliated 

 young; B, young colony; C, part of colony with polyps in extension (a) and contraction 

 (c); d, crenosarc; A, greatly, B, C, slightly enlarged. 



consists only of a base, with radiating ridges (sclerosepta} on the side to- 

 wards the flesh. These alternate with the septa (sarcosepta) of the polyp. 

 In most forms there is, in addition, a cup (theca) in the column of the 

 polyp, the sclerosepta extending inward from this. 



