THE ANTIQUITY OF THE ZOANTHID ACTHNIANS. 197 



closely related to the extinct group of corals known as the Rugosa or 

 Tetracoralla. The increase of mesenteries in the rugose polyps must have 

 followed exactly the method of increase characteristic of the zoanthids, 

 except that in the former the additions took place within four of the 

 six primary cliambers or exocoeles, whereas in zoanthids new members are 

 added within only two of the six primary chambers. I have also shown 

 that the single stomodaeal groove or siphonoglyph of the zoanthid polyp 

 finds its counterpart in the cardinal or ventral directive fossula of the 

 rugose coral.* 



As the type of mesenterial increase represented in ordinary actinians' 

 and modern corals indicates a common ancestry for these two gi'oups so 

 I conclude that the zoanthids and rugosa had another common ancestry, 

 that the zoanthids are the nearest skeletonless representatives of the 

 stock frOni which sprung the skeleton-forming Rugosa. 



If this be the true relationship of the rugose corals we shall have to 

 assume that the skeleton-producing habit has been acquired independ- 

 ently by two groups of zoantharian pohps, the Hexacoralla and the' 

 Tetracoralla; or, at any rate, that the two separated at the primary six- 

 rayed condition. This double origin, however, is much more likely to have 

 taken place than that the Hexacoralla have been derived from the Tetra^ 

 coralla, as is frequently assumed. From a knowledge of the development 

 and morphology of the two groups the zoologist can only conclude 

 that their common ancestry is represented only as far as the primary stage 

 with six septa and six pairs of mesenteries; they diverge from this, that 

 is, with the commencement of the metacnemic stage, along wholly dif- 

 ferent lines of development. 



According to Zittel (1. c, p. 80) the distribution of the Tetracoralla 

 in time is as follows : ''The typical Tetracoralla are confined to the 

 Palaeozoic rocks. They are unknown in the Cambrian, and make their 

 first appearance in the Ordovician, where they are sparsely represented 

 in North America and in Europe. . . . The period of maximum develop- 

 ment falls in the Silurian, which contains the largest number of genera 

 and species. . . . The Tetracoralla are not less conspicuous in the De- 

 vonian. . . . According to Freeh, the genera Gif/antosfylis, Pina- 

 coplii/llmii, and Coccophyllum, occurring in the Alpine Trias, belong to 

 the Tetracolla." 



No one disputes that the tetracorallid or rugose type of coral is wholly 

 extinct to-day. Odd living forms have been described for which such 

 affinities have been claimed (e. g. Moscleya, HaplopliylUa, Guynia) but 

 they have failed to justify expectations. Even the suggested extension of 

 range of the group beyond the Palaeozoic into the Mesozoic is quite recent. 



The zoanthids of to-day, therefore, find their nearest coral producing 

 ancestors away back in Palaeozoic times. Though their relationship with 

 the tetracorallids is not quite so close as that of the actinians with the 

 hexacorallids yet the method of mesenterial and septal formation shows 

 that they belong to a common stock, already differentiated to a certain 

 degree in very early Palaeozoic times. Among the early faunae in the 

 eartli's history there must have been at least these two well defined 



♦Impressed with this relationship it seems not too much to expect that forms of zoanthids may be 

 met within which new mespnteiies are added within four of the six primarv chambers, instead of 

 only two; and. on the other hand, I have sought for Tetracoralla in which new sepia were developed 

 in only two primary chambers, instead of four, though hitherto without success. 



