102 



MANUAL OP ZOOLOGY. 



external envelope, but it forms an axis, upon which the entire 

 actinosoma is spread. The actinosoma, in fact, is inverted, 

 and the * sclerobasis ' is secreted by the outer surface of the 

 ectoderm. The sclerobasic corallum is, therefore, truly ' out- 

 side the bases of the polypes and their connecting ccenosarc, 

 which, at the same time, receive support from the hard axis 

 which they serve to conceal.' (Greene.) Upon this view the 

 sclerobasis is termed ' foot-secretion ' by Mr. Dana. 



The ' sclerodermic ' corallum, on the other hand, is secreted 

 within the bodies of the polypes, apparently by the inner layer 

 of the ectoderm, the * enderon ' of Huxley and it is, there- 

 fore, termed ' tissue-secretion ' by Mr. Dana. A sclerodermic 

 corallum, of course, like the animal which produces it, may be 

 simple or composite, according as it is produced by a single 

 polype or by several united by a ccenosarc. It consists, there- 

 fore, of a single calcareous cup, or ' corallite ; ' or of several 

 such united by a common calcareous bond or basis, the 

 * ccenenchyma.' Taking a single ' corallite ' (fig. 27 d) as the 



a 



d 



Fig. 27. Morphology of Corals, a. Cup of Acercularia ananas, showing 

 calicular gemmation, enlarged, b. Diagram of Eugose Coral (Polycoelia 

 prof undo,), showing the quadripartite arrangement of the septa, c. Dia- 

 gram of a recent coral, showing the sextuple arrangement of the septa. 

 d. Vertical section of Campophyllum flexuosum, showing tabulae. 



type, we find that it shows its origin and nature plainly in its 

 form. It consists of a cylindrical or conical tube of carbonate 

 of lime, the outer wall of which is called the ' theca.' The 

 upper part of the space included by the ' theca ' is vacant, and 

 it is termed the cup or ' calice ' ; but the lower part is sub- 

 divided into a series of chambers, or ' loculi,' by a series of 

 radiating, vertical, calcareous plates, which are called the 

 * septa ' (fig. 27, &). The septa extend from the inner surface 

 of the theca towards its centre, where they usually unite to 



