112 Dr. J. E. Duerden on the 



intioducGS a difFerent fundamental type of metacnemic succes- 

 sion. The absence of directives and the irregular mesenteric 

 systems described in certain colonial corals — Lophophyllia, 

 Alussa, Euphyllia, &c. — are probably in all cases due to 

 asexual methods of reproduction, and do not indicate the true 

 characteristics such as would be revealed by polyps developed 

 directly from larva?. 



Similarly with modern Actinians, exclusive of Zoanthids 

 and Ceriantliids. Their metacnemic sequence conforms most 

 closely with that of modern corals. In many the metacnemes, 

 even of the first cycle, are incompletely established {Ed- 

 ivardsia, Gonactinia^ Oractls, Peachia), but so far as they go 

 they indicate no new type of development. Dr. O. Carlgren 

 (1897) has lately described an Actinian, Endocoelactis, the 

 adult characteristics of which seem to show that the second 

 and third cycles of mesenteries have been added in unilateral 

 pairs within the primary entocoeles, the directive entocoeles 

 excepted. The discovery is full of suggestiveness in con- 

 nexion with the entocoelic succession followed in Porites and 

 Madrepora. Carlgren, however, is inclined to regard tiie 

 arrangement as merely an exception to the general plan of 

 Hexactinian development, and places the genus under the 

 subtribe Actinin^e. 



The entocoelic growth of mesenteries and septa is also 

 difFerent from anything which has yet been definitely esta- 

 blished among fossil corals, including the Rugosa of Palaeo- 

 zoic times. It must be acknowledged that our acquaintance 

 with the septal succession of extinct corals is very incomplete; 

 a study of numerous types by means of serial sections will be 

 necessary before an adequate knowledge of their method of 

 growth is available. Thanks to the labours of R. Ludwig 

 (1862, 1865) and A. Kunth (1869), we are more or less 

 familiar with the septal plan of a large number of Rugose 

 corals, in so far as this can be determined from the ridges and 

 grooves exhibited on the outer surface of the coralla. Ac- 

 cording to Ludwig the primary plan in the Rugosa is hexa- 

 merous, whereas Kunth assumes a tetrameral condition both 

 for the primary and secondary stages. Both writers find the 

 nietasepta to be added successively at four regions, in a manner 

 which finds its expression in what is now known as Kunth's 

 law. 



Some of Ludwlg's descriptions and figures (pi. xxvii. 

 figs. 2) indicate that new septa were added within all the six 

 primary chambers, while some few septal pairs appear to 

 have been added axially, i. e. within an entoccele, as in 



