THE ANATOMY OP THE MADREPORARIA. 255 



is, as usual, oval in outline, measures about 18 mm. in the longer axis, 

 and 9-13 mm. in the shorter. Fresh polyps may be budded off from 

 the side, or, more rarely, from the calyx. 



The theca has the porous appearance characteristic of the Perforata, 

 and is marked on the external surface by distinct spinous costce or 

 ridges ; each of which corresponds externally to the attachment of a 

 septum on the interior surface of the theca (fig. 1 4). 



Both exosepta and entosepta occur in this form. Of true — i.e. 

 entoccelic — septa there are only three orders, with occasional traces of 

 a fourth ; from the sides of each primary and secondary entoseptum 

 grows out an exoseptum (fig. 14), and the relations of these two classes 

 to each other are rather complicated. Such a system as a-a in fig. 22 

 shows, in a transverse section taken high up in the polyp, the arrange- 

 ment diagrammatised in fig. 19, consisting of five true entosepta (each 

 of Avhich lies between a pair of mesenteries), and four exosepta alter- 

 nating with them. In a lower section (fig. 20), the two exosepta 

 which grow out from the sides of adjacent primary and secondary 

 entosepta, fuse over and with the intermediate tertiary septum into 

 one. Lower yet (fig. 21) the two compound septa thus produced in 

 each system meet over and with the secondary septum, so that the 

 columella is due to the irregular fusion (fig. 15) of twelve primary 

 entosepta, distinct for their whole length, and twelve other septa thus 

 elaborately compounded. 



ii. Anatomy. — In Ehodopsammia, which, like all the other forms as 

 yet described, bears a close resemblance to an Actinia, the mouth-disc, 

 unlike the case in Flabellum, passes into a distinct external body ivall 

 of ectoderm, mesoderm, and endoderm (extending in some specimens 

 very much further down than is represented in the diagram, fig. 13). 

 Between this and the theca lies a narrow space in which run, parallel 

 to the long axis of the corallum, lamellae of tissue, connected on the 

 one hand with this external body- wall, on the other with the tissues 

 clothing the exterior surface of the theca (figs. 13, 14, 17, M'). These 

 lamellae correspond externally to the attachments of the mesenteries 

 on the interior surface of the theca, and are apparently continuous 

 with them over the lip of the calyx (fig. 13). They thus divide the 

 space between body-wall and theca into a series of long chambers, 

 corresponding to the exocoeles and entocosles, in each of which lies a 

 costa. Between these chambers and the exocceles and entocosles a 

 system of ramifying canals permeates the theca, placing the two sets 



