On tJie Stmctave and Relations of certain Corals. 155 



cups of hard tissue fitted inside the older tubes and calicles. In 

 Stjrinf/ojiora this condition of the tabulae is much more marked, 

 and the corallum appears as if formed of a series of calicles fitted 

 one within another. 



A difficulty appears to arise from the peculiar mode of the 

 development of the calicles by budding in Heliopora, the foldings 

 of the walls of the calicles being due, to a considerable extent at 

 least, to the formation of these walls from a circle of coenenchymal 

 tubes. The septa are, however, not entirely formed in this way. 

 It would of course be of great interest to see whether the pri- 

 mitive calicle, in the developing Heliopora colony, forms calcareous 

 septa. 



Heliopora having so commonly twelve septa, and in conjunction 

 with these eight mesenteries, it was at first thought that here 

 , some key would be found to the elucidation of the question of the 

 relations of the tetrameral corals to the Hexactinians ; but no defi- 

 nite arrangement of the eight mesenteries to the twelve septa 

 could be discovered. Ludwig and Pourtales have concluded that 

 the tetrameral condition in the Rugosa is the result of a modifi- 

 cation of an originally hexameral arrangement — that the Rugosa 

 are, in fact, modifications of the Hexactinian type. Kunth, however, 

 using similar methods, has come to an opposite conclusion. Now 

 that it is known that an Alcyonarian exists which constructs a 

 solid calcareous corallum, in histological structure scarcely, if at 

 all, to be distinguished from that of many Madreporaria, and 

 that this Alcyonarian also possesses marked calcareous septa, 

 which septa show, notwithstanding the octameral arrangement of 

 the mesenteries, a hexameral disposition in being often twelve in 

 number, it seems that the question of the affinities of the Rugosa 

 may fairly be reopened. The presence of well-marked calcareous 

 septa in Cryptohelia and other Stylasteridse (which septa are 

 equal to the tentacles in number, but nevertheless to be regarded, 

 like those of Heliopora, as pseudo-septa) is significant. The 

 marked tetrameral arrangement of the septa in Rugosa, and the 

 presence in many forms of tabulae, are certainly characters not 

 opposed to the alliance of these corals with the Alcyonarian s ; and 

 the fact that paired series of opercula occur in certain Rugosa, 

 which are compared by Lindstrom, their discoverer, to the skeletal 

 structures of certain Primnoce, seems to be evidence in favour of 

 such an alliance of the very strongest kind. In no Madreporaria 

 do paired hard structures, at all resembling those of Primnoce or of 

 Goniophijllum pyramidale, occur. The opercular structures in 

 the coralla of C ryjytohelia and Lcpidop>ora can scarcely be regarded 

 as comparable with the opercula of Rugosa. The structures are 

 merely folds of the lip of the calicle, and are continuous with it 

 and immovable, not movable separate articulate structures. Many 

 Rugosa show an arrangement which may well be compared to the 

 distinction of dorsal and ventral regions in Alcyonaria. The most 

 important distinctive character of the Rugosa appears to be the 



