Mr Menon, Notes on Semper' s Larvae. 415 



mesentery stage thus forms the starting-point of the genera of 

 the Zoantheae. 



I have not obtained later stages than this. There can be no 

 doubt, however, that new mesenteries are formed, as stated by 

 McMurrich, in the two mesenteric chambers lying on the two 

 sides of the ventral chamber. 



In the development of the Zoantheae, then, we have a sterro- 

 gastrula in which a central cavity is developed by the breaking 

 down of the inner portion of a syncytium. Two mesenteries are 

 developed at first, dividing the coelenteron into a dorsal and a 

 ventral chamber, and in each of these two new mesenteries make 

 their appearance. There is thus a stage with six mesenteries and 

 six mesenteric chambers. Thus far the order of development of 

 the mesenteries is the same as what is usually described for other 

 Actiniaria. Whether this stage is only a transient stage, rapidly 

 passing into the next, can be best determined by direct observa- 

 tion. The difference in the size of the macromesenteries suggests 

 that the dorsal and ventral pairs must have been formed at about 

 the same time and not long after the lateral ; the micromesen- 

 teries too were presumably formed within short intervals, and this 

 seems to be the actual case, as the specimen with ten mesenteries 

 does not differ very much in size from the smallest specimen with 

 twelve mesenteries. On the other hand, there is a considerable 

 difference with regard to size between the specimen with six 

 mesenteries and that with ten. Though these inferences cannot be 

 of much value, based as they are on solitary examples, they render 

 it very probable that there is a considerable interval between the 

 six- mesentery stage and the next. The macromesenteries were 

 most probably formed at a very early stage — this is actually the 

 case with the lateral macromesenteries as seen in Zoanthella. 

 The micromesenteries are not formed till the larva has reached a 

 much more advanced stage, and then they are formed pretty 

 much at the same time. 



The development of the dorsal pair of micromesenteries would 

 convert the six-mesentery stage into the Edwardsia stage. While, 

 according to Van Beneden, they arise after the dorso-lateral pair 

 of micromesenteries, my sections induce me to believe that they 

 are the last of the three pairs to be formed. In any case they are 

 not the first ; so that even if an eight-mesentery stage were to 

 turn out to be of some duration, it would not be an Edwardsia stage. 

 On the whole it is reasonable to conclude that the charac- 

 teristic stage in the development of the Zoantheae is the stage 

 with six macromesenteries, and that this stage is as characteristic 

 of the group as the Edwardsia stage is of the Hexactiniae. 



With the twelve-mesentery stage we practically reach the 

 adult microtype. 



