THE ANATOMY OP THE MADREPORAKIA. Q 



and mesoderm which clothe the exterior surface of the theca ; 

 and into the canals thus formed project the costse. In 

 M. Durvilleij the layer of endoderm and mesoderm which 

 is immediately apposed to the exterior surface of the corallum, 

 rises in a ridge towards the external body wall; and at the 

 points where these layers meet and fuse are formed the costse, 

 i. e. in the angle of the mesoderm ; and therefore between the 

 costse lie the canals. A comparison of fig. 4 with (4) fig. 17 

 will make clear the anatomical difference. 



There is thus no trace of any structure resembling the '' peri- 

 pheral continuations of the mesenteries of v. Koch.^' 



These canals appear to open over the lip of the calyces into 

 the polyp cavities ; they are connected with each other trans- 

 versely between the spikes (echinulations) of the costse (figs. 

 6, 7); and further, by radial canals (figs. 3, 4, c^.) they open 

 into the internal longitudinal canals, which I believe, as above 

 stated, to have, at an earlier period in the history of the branch, 

 occupied a position similarly external to the corallum. The 

 whole system which thus perforates the corallum, and allows 

 free current of fluid to even the most remote parts of the 

 colony, is lined by endoderm and mesoderm throughout, and 

 opens into similarly lined polyp cavities. 



The general structure of the colony is, therefore, (1) an 

 external body wall, under which and between the costse 

 lies (2) a series of external longitudinal canals opening 

 into each other, and also through the corallum, into (3) the 

 internal canals, mainly longitudinal, with radial and trans- 

 verse connections, communicating in their turn with (4) the 

 coelentera of the polyps. Into the last the external 

 longitudinal canals also open directly, through the theca. The 

 whole system is of course merely a complication of the primi- 

 tive coelenteron. 



Of the polyps there are at least two distinct types, which 

 are full of interest as constituting the first record of marked 

 dimorphism among the Madreporaria. Both are Actinian in 

 structure. 



Type A has in the highest sections twelve perfectly normal 



