MORPHOLOGY OF THE MADREPORARIA. 43 



So far as concerns modern corals I have lately shown (BioL. 

 BULL., July, 1904) that the septa, correlatively with the mesen- 

 teries, present very marked bilateral phases during nearly the 

 whole of their ontogeny ; it is only as the organs attain maturity 

 that they become radially symmetrical. 



Likewise the study of the development of a large number of 

 rugose corals by the method of grinding proves that they are all 

 characterized by a bilaterality of growth after the protoseptal 

 stage ; in the majority of species the bilaterality passes into a 

 radial stage, but in some it is retained at maturity. The bilater- 

 ality of the Rugosa is however of an altogether different nature 

 from that of modern Madreporaria ; the septa in rugose corals 

 are added as bilateral pairs at only four regions of growth, two 

 on either side, whereas in modern corals the additions are made 

 all round the periphery within the six primary interseptal spaces. 

 Notwithstanding these developmental differences a more or less 

 radial symmetry is reached by the adults of both groups. Little 

 or no importance is thus to be attached to the old distinction 

 that the Rugosa are bilaterally symmetrical while recent corals 

 are radial ; the species of both are ontogenetically bilateral, and 

 the degree of radiality attained by the adult varies much in forms 

 otherwise closely related, or even in individuals of the same 

 species. Developmental bilaterality and mature radiality are 

 just as much a feature of the extinct Tetracoralla as they are of 

 recent Hexacoralla. 



The more or less perfect radiality of the different groups of 

 the ccelenterates as a whole is reached from very different devel- 

 opmental conditions, as can easily be seen from a survey of what 

 is known with regard to the early stages of the organs in the 

 scyphomeduss, actinians, zoanthids, cerianthids, and recent and 

 fossil corals. It follows that the adult radiality of organs which 

 arise in such widely divergent manners in no way implies mor- 

 phological relationship among the animals possessing them. 

 Rather, it is an adaptation to the uniformity of environmental 

 influences on all sides to which sessile or floating organisms are 

 subject. The manner in which the organs arise seems imma- 

 terial ; however varied the origin the same end is attained at 

 maturity. The feature in the Rugosa here sought to emphasize 



