D^ 



CORALS 



there are about twelve large septa alternating with twelve 

 smaller septa. The larger septa are usually slightly exsert, 

 and are continued over the lip of the theca into twelve 

 costae, which are extended on to the peritheca to meet the 

 costae of neighbouring calices. 



In most examples there is a trabecular columella to 

 which the larger septa are joined, but this structure is 

 rudimentary in others. 



If a large colony be carefully examined some calices 

 will be found more elongated than the rest and show a 

 constriction which indicates a division of the calices into 

 two equal or unequal portions. This may be taken as a 

 sign that the usual method of increase in the number of 

 polyps is by the process of splitting into two or by 

 fission. 



The process of fission is brought about in these corals 

 by the increase in one diameter of the calyx accompanied 

 by an increase in the number of septa, and this is followed 

 by a constriction of the calyx wall in a plane at right 

 angles to the diameter which has increased in length 

 and the constriction is continued until the single calyx is 

 divided into two calices. The fission of the coral polyp is 

 on the same plan as that of the calyx, an increase in the 

 number of the mesenteries being followed by a vertical 

 plane of constriction of the body wall of the oral disc and of 

 the crown of tentacles ; and finally, the division of the 

 polyp vertically into two polyps. 



This method of asexual reproduction of the individuals 

 of a colony of Favia is undoubtedly the most common, but 

 it is not the only one, because at the base or, in the encrust- 

 ing forms, at the growing outside edge of some specimens 

 small calices may be found arising from the coenosteum 

 between the other calices. Increase in numbers of in- 

 dividuals, therefore, may occur not only by fission but also 

 by gemmation in this genus. 



In the anatomy of the polyps of Favia there is one point 

 of special interest to which attention should be drawn. 

 In the polyp of Galaxea, as alread\' mentioned, there are 

 two pairs of directive mesenteries as in most of the sea- 



