STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 219 



canals themselves may exhibit elongation and concentric 

 arrangement around these canals, but not to the same degree 

 as in the chamber diaphragms. 



The chamber diaphragms, then, I regard as being composed 

 of eudodermal muscular cells, formed by modification of the 

 ordinary endodermal pavement cells. Whether they consist of 

 one or two layers of these cells is doubtful; probably of two layers, 

 with a small amount of gelatinous ground substance between. 



It thus appears that, if Minchin is right in regarding the 

 muscular cells of the oscular diaphragm in Leucosolenia as 

 ectodermal, and if Schulze is right in regarding the lining of 

 the exhalaut canals in Heterocoele sponges as endodermal, then 

 both the ectoderm and endoderm may give rise to strikingly 

 similar muscular structures. Possibly this may be an argument 

 for regarding the muscular cells of the oscular diaphragm in 

 Leucosolenia as endodermal and not ectodermal; but it 

 appears to me that this is a question which cannot be decided 

 in the present state of our knowledge. 



Concerning the remaining endodermal cells, the collared 

 cells which line the flagellated chambers, I have little to say. 

 I may, however, call attention to fig. 25, representing half of 

 a flagellated chamber of an undetermined species of Leu- 

 candra, in which Sollas's membrane and the collars of the 

 collared cells are remarkably clearly seen. This figure also 

 shows the exhalant aperture with its membranous diaphragm, 

 and two prosopyles. Whether or not Sollas^s membrane is 

 continuous over the prosopyles I have been unable to deter- 

 mine, and it appears to me that the point can only be decided 

 by the examination of living specimens. 



I quite agree with Bidder (21) that the endoderm is not only 

 multiform, but also '' most proteic," as, indeed, was long ago 

 shown by Carter (30) from observations on the living Sycon 

 (Grantia) compressum, in which he observed the collared 

 cells becoming amoeboid, and moving about the field of the 

 microscope. I am therefore quite prepared to believe that the 

 existence of Sollas's membrane may be only a transitory con- 

 dition, due to special circumstances, of which we are as yet 



