152 McMURRICH. [Vol. V. 



A second offset arising from the main line, or even from this 

 larval stage, gives rise to the Zoantheae. In this group we seem 

 to have a localization of growing regions similar to what is indi- 

 cated in the Ceriantheae ; but the Zoantheae possess two such 

 regions, situated on each side of the ventral directives, new 

 mesenteries continually forming at this point, and just as con- 

 tinually being divaricated towards the dorsal surface. We find, 

 consequently, a very different condition as regards the relative 

 position of the primary (including here under this term mesen- 

 teries V and VI) and secondary mesenteries from what was 

 pointed out as occurring in the Ceriantheae. The primaries are 

 divided into two groups by the secondaries, the dorsal group 

 containing ten mesenteries, and the ventral only the ventral 

 directives. Furthermore, the youngest secondary mesenteries 

 are situated nearest the ventral surface. 



A third offset from the main line occurs soon after the de- 

 velopment of this stage, and leads to such forms as Scytop/iorus, 

 in which, in addition to the twelve primary mesenteries, we 

 have a single mesentery present on each side of the dorsal 

 directives. It is noticeable, however, that all the fourteen 

 mesenteries of ScytopJiorus are perfect, and it may be possible 

 that it has arisen from the main line much lower down, that is, 

 after the Halcampa condition had been established. The loca- 

 tion to which I have assigned it seems to me preferable, how- 

 ever, and I see no difficulty in the way of supposing that the 

 mesenteries V, VI, and VII have become perfect since the sep- 

 aration from the main line. 



A fourth offset leads to Gonactinia, which presents a stage in 

 advance of Scytophorns. It retains the imperfection of V and 

 VI, but has in the dorso-lateral intermesenterial space on each 

 side a pair of imperfect mesenteries. 



A little further down, and we reach Oractis, with two pairs of 

 imperfect secondary mesenteries in the dorsal and lateral inter- 

 mesenterial spaces ; mesenteries V and VI being imperfect as 

 in Gonactinia. In the diagram I have represented the last two 

 groups and Scytophorns as far removed from the main line as 

 the Zoantheae. It seems probable, however, that they really 

 stand very close to it, and indicate the gradual acquisition of 

 new secondary mesenteries, which led to the formation of the 

 second cycle mesenteries of the Hexactiniae. It is an interest- 



