414 Mr Menon, 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- 



