246 CCELENTERATA. 



the calice, the visceral chamber is subdivided into a number of 

 vertical compartments (" loculi ") by a series of upright calcareous 

 partitions or " septa," which spring from the inner surface of the 

 theca, and are directed inwards towards the centre. The septa are 

 calcifications formed within the intermesenteric chambers (fig. 120, 

 sj>), so that each septum is placed between two mesenteries and 

 underneath a tentacle, and the total number of the septa is equal 

 to that of the mesenteries. The septa likewise increase in number 

 with the increasing growth of the polype, as the mesenteries do ; 

 and, like the latter, they vary in their width, so that they are often 

 spoken of as " primary," " secondary," and " tertiary " septa. 



The septa of both recent and fossil corals, when examined in cross- 

 sections, commonly show a composition out of two lamellae of dense, 



Fig. 126. — Caryophyllia borealis. A simple sclerodermic Coral, twice the natural size. 

 (After Sir Wyville Thomson.) 



usually light-coloured sclerenchyma (" stereoplasma"), separated by a 

 median, generally dark line (fig. 127). 



This central dark line (the " Primarstreif " of the Germans) has been 

 regarded as a mere line of calcification ; but it seems to be really a dis- 

 tinct median lamella, representing the primordial septum, while the 

 lateral layers of stereoplasma are of secondary origin. In some cases 

 (as in Caryophyllia borealis, fig. 127, a), it appears that this central plate 

 is itself double, and it is probable that it is always so in origin, even 

 though its component elements become completely fused in process of 

 growth. 1 In other cases (as in Heliophyllum and its allies), the septa 



1 Hinde has clearly shown that the primary lamella of the septum is really 

 double in the genus Septastrcea, and a somewhat similar structure of the septa 

 seems to obtain in the recent genus Flabellum. 



