E. MARSHALLr. — FLAGELLATED CHAMBER. 137 



defined, without however showing the slightest indication of the 

 presence anywhere of a limiting membrane. F. E. Schulze, as 

 he described the structure of the Hexactinellidan chamber- wall 

 for the first time, evidently believed that the choanocytes rested 

 on a continuous basal membrane, the outer (incurrent) surface of 

 which was furthermore assumed to be lined by a pavement- 

 epithelium. This was a mistake. Schulze ('99a, p. 209 ; 19' a, 

 p. 98) himself has been led by liis recent researches into the 

 histology of Schaudinnia arctica to the conviction that the mem- 

 brane has no existence, and that the presence of the pavement- 

 epithelium is questionable. I think it may be considered as a 

 settled question that there exists no special layer of any kind 

 outside of, and in contact with, the reticular layer of the choano- 

 cytes. Schulze (I.e.) has expressed the opinion that the 

 fundament on which the latter layer lies, should be considered 

 to be a relatively wide-meshed network of certain trabecuhe. I 

 should rather say, as I have already said above, that all the 

 trabeculse coming to the reticular membrane simply find inser- 

 tion in this for the ends of their fine dentritic branches. Some 

 of these terminal branches are indeed seen to creep along tlie 

 outer surface of the reticular membrane shortly before they 

 terminate ; but such occurrences in the species studied by me are 

 decidedly too few and far between to be regarded as giving a 

 ' Grundlage ' to the choanocyte layer. 



The nucleus, whose shape and position have already been 

 described, is plainly visible under a high magnifying power. It 

 measures only 1.5-1.7 ,"■ in diameter as seen in the surface view 

 of the chamber-wall. A fine nuclear membrane seems to be 

 present. The contents are nearly homogeneous or at most finely 



