1875.] Structure and Relations of certain Corals. 67 



opposite the lateral "Ventralfach." In Alveolites the one tooth is ventral 

 instead of dorsal. In Syringopora the septa seem to be very much of 

 the same nature as in Heliopora ; and in Heliopora, as already described, 

 the tabulae are not merely transverse floors, but the bottoms of cups of 

 hard tissue fitted inside the older tubes and calicles. In Syringopora 

 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 develop- 

 ment of the calicles by budding in Heliojpora, the foldings of the walls 

 of the calicles being due, to a considerable extent at least, to the forma- 

 tion of these walls from a circle of ccenenchymal tubes. The septa are, 

 however, not entirely formed in this way. It would of course be of 

 great interest to see whether the primitive 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 definite arrangement of 

 the eight mesenteries to the twelve septa could be discovered. Ludwig 

 and Pourtales have concluded that the tetrameral condition in the 

 Eugosa is the result of a modification of an originally hexameral arrange- 

 ment — that the Eugosa are, in fact, modifications of the Hexactinian type. 

 Kunth, however, using similar methods, has come to an opposite con- 

 clusion. Now that it is known that an Alcyonarian exists which con- 

 structs 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 Eugosa 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 Heliqpora, as pseudo-septa) 

 is significant. The marked tetrameral arrangement of the septa in Eugosa, 

 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 Eugosa, which are com- 

 pared 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 re- 

 sembling those of Primnoce or of Goniophyllum pyramidale, occur. The 

 opercular structures in the coralla of Cryptohelia and Lepidopora can 

 scarcely be regarded as comparable with the opercula of Eugosa. The 

 structures are merely folds of the lip of the calicle, and are continuous 

 with it and immovable, not movable separate articulate structures. Many 



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