415 
Mr Menon, Notes on Semper’s Larvae. 
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. 
