414 
Mr Men on, Notes on Semper s Larvae. 
The mesenterial filaments are well developed and differ from 
those of Zoanthella. They are much convoluted, so that in trans- 
verse sections the ectodermal portion is seen, sometimes in the 
form of kidney-shaped bands closely pressed against the middle of 
the free edge of the mesentery, sometimes lying on one side or the 
other, and often broken up into parts separated by intervening 
-regions of endoderm cells. In young specimens, however, the 
filaments have the same rounded outline as in Zoanthella. 
The youngest specimen of Zoanthina has only six mesenteries. 
They are all complete and have mesenterial filaments. The fila- 
ments are simpler than in the stage described previously, the 
mesentery ending in a simple, rounded border. The lateral 
mesenteries are much larger than the dorsal and ventral pairs, 
which differ from each other only slightly. The six mesenteric 
chambers are approximately all equal. Fig. 10 represents a 
section of this stage and indicates the probable order of the 
development of the mesenteries. The mesenteries and chambers 
are quite well-developed round the stomodaeum. The stomo- 
daeum is almost rectangular in section, and is not thrown into 
folds. 
In the next stage there are, as in the earliest, six complete 
mesenteries with mesenterial filaments. In sections passing 
through the stomodaeum, as well as for a distance below it, and in 
sections passing through the aboral region, these six mesenteries 
and these alone are present. But in sections of the intermediate 
region four small and incomplete mesenteries are also present. 
The dorsal and ventral primary chambers have no micromesenter- 
ies, but each of the remaining chambers has a micromesentery in 
it. The dorso-lateral pair of micromesenteries are slightly larger 
than the ventro-lateral pair and must therefore have preceded 
them in development. Fig. 11 is a diagrammatic representation 
of this stage. The parietal endoderm is richer in yolk than in 
the youngest larva. The mesogloea is thicker, especially in the 
mesenteries. 
In the next stage there are twelve mesenteries, having the 
arrangement shown in fig. 12. Two new micromesenteries have 
appeared in the dorsal chamber, thus bringing the number of 
mesenteries up to twelve. When it is remembered that there is 
a considerable difference in size between the smallest and the 
largest specimens of this stage, and almost all the specimens 
obtained belonged to this stage, there can be no doubt that the 
twelve-mesentery stage is a prolonged and the most characteristic 
stage in the life-history of this form. This receives additional 
interest from the fact that comparative anatomy long ago reduced 
the mesenterial arrangements of the Zoantheae to two funda- 
mental types, each consisting of twelve mesenteries, The twelve- 
