244 



CCELENTERATA. 



In other cases, as in the Red Coral (Cora/Hum), the spicules 

 become fused so as to form a solid cylindrical calcareous axis, which 

 occupies the centre of the ccenosarc (fig. 124). Such a ccenosarcal 

 axis is produced independently of the polypes of the colony, and 

 constitutes what is known as a " sclerobasic " corallum. The form 

 of the sclerobase or axial corallum is simple or branched, in accord- 

 ance with the undivided or divided condition of the ccenosarc ; and 

 its precise structure is very variable. In Corallium it is formed of 

 microscopic spicula united by a general calcareous matrix ; but in 

 other cases the uniting matrix may be horny, while in the Anti- 



*%\SMs^ 



Fig. 124. — A portion of a colony of Red Coral (Corallium rubruvi), longitudinally divided, 

 and having part of the ccenosarc with its embedded polypes removed. (After Lacaze-Duthiers.) 

 co, Ccenosarc, with its embedded polypes, its outer portion traversed by reticulate canals ; s, Sclero- 

 basic corallum, grooved for the reception of a series of longitudinal ccenosarcal canals (ca) ; p p, 

 Polypes having their tentacles (f) protruded ; cc, Gullet ; /' p' , Polypes retracted within the 



ccenosarc. 



pathidce and in many of the Gorgonidce the sclerobase is not formed 

 by the fusion of calcareous spicules, but is the result of the secretion 

 of horny matter in successive concentric layers. 



In the great majority of the coralligenous Actinozoa the corallum 

 is calcareous, and is not formed by the fusion of definite " spicula," 

 but is the result of the secretion of carbonate of lime by the outer 

 surface of the ectoderm. Such a corallum has been commonly 

 spoken of as " sclerodermic." The actual calcareous tissue (" scler- 

 enchyma") which constitutes an ordinary Madreporarian coral has 

 a peculiar microscopic structure, appearing as if formed of bundles 

 of minute calcareous fibres, and commonly having a sub-crystalline 

 character. When these bundles of fibres are cut across transversely 

 (fig. 125, a), they give rise to a characteristic stellate structure, which 

 in some cases (e.g., Stylophora, Pocillopora, &c.) is of a very regular 



