STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 27 



instance connecting the gland-cell with the epithelium, but it is 

 very unusual to see so many. Just on the right of the gland- 

 cell the epithelium is raised up by a projecting spicule which 

 does not itself appear in the section. On the dermal surface 

 of the sponge the gland-cells are more like ordinary amoeboid 

 cells immediately underlying the epidermis. 



The essential agreement of these cells with those described 

 by von Lendenfeld in, for example, Dendrilla cavernosa 

 (19) is obvious, both as regards structure and arrangement. 

 According to this author the cells secrete a cuticle which is not 

 to be distinguished from the spongin of the horny fibres. I 

 agree that they probably secrete a cuticle, but, as Vosmaer 

 points out (20), no grounds are given for supposing that this is 

 identical with the spongin, and the fact that similar gland-cells 

 occur in the Calcisponges argues against the assumption. 

 Very possibly the cuticle, after a time, more or less entirely 

 replaces the original epithelium, and not only, as suggested by 

 von Lendenfeld,^ so much of it as may have been accidentally 

 damaged. Hence, perhaps, arises the difficulty of making 

 out the structure of the surface epithelium. 



Endothelial Cells. — The embryos, which I have described 

 elsewhere (10), lie each in a separate cavity in the mesoderm 

 around the flagellated chambers. These embryo-containing 

 cavities (fig. 38, em. c.) are lined each by a single layer of 

 pavement-cells, which are not to be distinguished in my 

 sections from the pavement-cells already described. These 

 cells I regard with Schulze (11) as being of mesodermal origin, 

 and hence endothelial. 



In my memoir on Stelospongos flabelliformis (2) I 

 regarded the embryo-containing cavities in that sponge as pro- 

 bably, though by no means certainly, specialized parts of the 

 exhalant canal system. As the result of further study, how- 

 ever, I must relinquish this view, and regard the cavities as 

 special excavations in the mesoderm, and the remarkable giant 

 pavement-cells which line them as of mesodermal origin. 



^ For an abstract of von Lendenfeld's observations on the subject 

 vide 20. 



