No. i.] MORPHOLOGY OF THE ACThVOZOA. 155 



phenomenon cannot be regarded as degeneration, but is rather 

 an arrest of development. I imagine that the Halcampas with 

 secondary mesenteries are phylogenetically the older, and those 

 with only primary mesenteries have been derived from them by 

 an arrest of the development of the secondaries. 



With regard to PeacJiia I have already indicated what I 

 believe to have been its origin (p. 139). There is a possibility, 

 however, that the forms which are to be assigned to this genus 

 may have arisen from the main line of descent towards the Hex- 

 actinian type, before the radial symmetry had replaced the more 

 primitive bilaterality. I think, however, that this idea is nega- 

 tived by the following consideration. 



It has been shown that the pairs of secondary mesenteries 

 which are present in Peachia, occupy the ventro-lateral and 

 lateral intermesenterial spaces ; that is to say, they correspond 

 to the mesenteries of Halcampa which I have indicated in the 

 diagram as IX and VIII. It seems probable that the order of 

 appearance of the mesenteries of the second cycle in the phylo- 

 genetic development is that indicated by Gonactinia and Oractis, 

 and this is confirmed by the observations of the Messrs. Dixon, 

 already referred to. Consequently, it appears that the Peachias 

 have arisen from forms which possessed a complete cycle of 

 secondary mesenteries, one pair of which, namely, the dorso- 

 lateral pair, they have lost. Their affinities seem to be with 

 the Halcampidae, and their ancestors are probably represented 

 more or less closely by those Halcampas which possess a cycle 

 of imperfect secondary mesenteries. 



To complete the discussion of the phylogeny of the Actino- 

 zoa, it still remains to consider the Madreporaria, the Alcyonaria, 

 the Rugosa, and the Antipatharia. 



From the observations of Von Koch, Von Heider, Bourne, 

 and Fowler, we now know that the Madreporaria are constructed 

 upon essentially the same plan as the Hexactiniae. The mesen- 

 teries are arranged in pairs, and hexamerously, and the longitu- 

 dinal muscles are on the contiguous faces in each pair except in 

 the cases of the two pairs of directives, which, however, in some 

 species may be absent. I believe that one will not err very much 

 in regard to the relationships of the Madreporaria in accepting 

 the statement of R. Hertwig ('82) : " Most corals will doubt- 

 less be placed later on with the Hexactiniae ; perhaps a natural 



