DEVELOPMENT OF AGARICIA. 503 



smaller than the filaments of mesenteries I and II. The filament 

 on the left of the larva (right in the sections) extends to about section 

 73 (Fig. 4, E), that on the right of the larva to about section 77, the 

 limits not being sharply defined. 



The filaments of mesenteries /// differ histologically from those of 

 I and //. The ciliated cells are shorter and thicker and gland cells 

 seem to be absent, resembling in this the endoderm cells, from which 

 the filament is not always sharply to be distinguished. 



A table is given showing the extent of the six primary mesenteries 

 in this larva. 



Aboral end section No. 11.5 112 102 104 112 102 



Only mesenteries I to IV are complete. 



This table shows that all of the mesenteries extend practically the 

 whole length of the larva. The third and fourth mesenteries are, 

 however, considering their early appearance in development, some- 

 what shorter than the others. The mesoglea of the mesenteries is very 

 thin and in many cases can be seen only where it is cut obliquely. 

 That it is somewhat tough and rigid, at least in fixed and stained 

 preparations, is shown by its being slightly displaced in sections, 

 leaving the less resistant entoderm cells behind it. 



The gastrovascular cavity is clearly in the process of being divided 

 into compartments corresponding to the mesenteries. It will be 

 noticed in Figure 4, E and F, that a cleft in the entoderm occurs on 

 either side of each mesentery and that entoderm bulges inward be- 

 tween the mesenteries and between these clefts. This would seem to 

 indicate that the formation of the compartments is brought about 

 by the formation of the mesenteries. There is evidence in the histol- 

 ogy of the entoderm that the clefts referred to are formed by the 

 separation and breaking down of cells rather than by a process of 

 infolding. This would tend to support the theory that the clefts 

 are due to the closer adherence of the contiguous entoderm cells to 

 the more rigid mesoglea of the mesenteries. 



