POSTLARVAL DEVELOPMENT. 83 



the first cycle of six pairs of mesenteries " protocnemes," and all subsequent 

 mesenteries " metacnemes," whether becoming arranged in cycles or not. 

 The protocnemic stage is practically the same for all the different groups of 

 Actiniaria and Madreporaria. It is in the metacneniic stage that distinguish- 

 ing characteristics are introduced, development in all cases proceeding in a 

 manner altogether different from that of the protocnemic stage. The interval 

 is thus one of the greatest significance in the phylogeny of the Anthozoa, as 

 well as in the ontogeny of the individual polyp. 



The metacneniic development here established for Siderastrea is that 

 most usual for ordinary anemones and corals ; new isocnemic pairs appear in 

 the six primary exocceles all round the polypal wall. In the group of the 

 Zoantheae the metacnemes appear at only two restricted regions, one within 

 the exoccele on each side of the ventral directives, and each unilateral pair 

 consists of a large and a small member (anisocnemic). In the Ceriantheae 

 the metacnemes appear as bilateral pairs at only one region, within what 

 seems to be the entocoele of the ventral directives. Furthermore, the metac- 

 nemes in the Zoantheae and Ceriantheae do not form one or more cycles 

 distinct from the protocnemes (monocyclic), as in Siderastrea, most other 

 modern corals, and ordinary anemones (polycyclic). 



These differences in the origin of the metacnemes separate in the clearest 

 manner the zoanthids and cerianthids from ordinary actinians and corals. 

 On the other hand, the metacneniic similarity in actinians and madreporar- 

 ians proves that the two groups are much more closely allied to one another 

 than to any other group ; the only important difference between the two 

 consists in the presence or absence of a calcareous skeleton. 



In general, it will be found that in the Actiniaria and Madreporaria the 

 organs as a whole beyond the protocnemic stage develop successively from one 

 border of the polyp to the other. The adult cyclic arrangement is clearly a 

 later modification of a primary dorso-ventral plan ; in some instances, however, 

 the cyclic tendency strikes back, as it were, to the first appearance of certain 

 of the organs, for the prototentacles and protosepta usually arise a cycle at 

 a time. The second-cycle septa in S. radians are also interesting in this 

 respect, for in some polyps the six members of the cycle appeared simulta- 

 neously, but in others in successive pairs (p. 87). 



THIRD CYCLE OF MESENTERIES. 



The polyps reared from the larva were not kept alive beyond the com- 

 pletion of the first and second cycles of mesenteries ; hence for what follows 

 as to the order of appearance of the third cycle recourse will be had to 



