E. MAESHALLI. — FLAGELLATED CHAMBER. 131 



open gaps in it ; however, since its situation is such that the 

 current of water caused by the flagella in the chambers could 

 scarcely exercise an unequal pressure on it in either direction, 

 there should be practically a standstill of water at the gaps 

 under normal circumstances. On both of its surfaces the 

 membrane furnishes points of insertion for several trabeculse. 

 As to its morphological nature, I believe in its identity with the 

 trabeculse. It seems to consist of trabeculse simply spread out 

 in a film-like manner, just like certain parts of the dermal, the 

 gastral or the canalar membrane. The thin protoplasm looks 

 exactly like that of any local trabecular expansion ; the nuclei, 

 met with at long and irregular intervals, are just the same in size, 

 appearance and staining capability as those of the trabeculse. 

 Moreover, where several gaps lie close together (as on the left- 

 hand side of fig. 22), the thin beams left between them are in 

 no way distinguishable from the ordinary trabeculae. 



Turning our attention to the chamber-wall itself, this con- 

 sists par excellence of the reticular membrane formed by the 

 choanocytes and of a narrow and fihny rim around the apopyle, 

 which rim I will call the marginal membrane. Let the latter be 

 first treated of here. 



The marginal membrane (PI. IV, fig. 22 tfe PL V, fig. 39 ; m.m.) 

 is but another structure which is to be considered as identical in 

 nature with the trabeculse. It is in no distinguishable feature 

 different from the connecting membrane. Like this it is occa- 

 sionally fenestrated and where the gaps occur close together, the 

 appearance is exactly like that of a trabecular cobweb. The 

 protoplasm is seen at places to be directly continuous with the 

 trabecular arising from or inserted in it (figs. 22 & 39). The 



