E. JIAKSUALLI. TRABECULE. l-vl 



structural conception to the relatively thicker parts of the 

 trabeculie. 



To be plain, my notion is that for once among the sponges 

 we find in the Hexactinellids that the development of pinacocytes, 

 both as an investing of the exterior and a lining of all the 

 internal cavities and passages, is entirely suppressed. Tliis 

 seems to be in harmony with a certain point in the structure of 

 tlie chamber-wall as described by me, vl~., that the convex 

 outer (incurrent) surface of this is, so to speak, naked, the basal 

 ends of the choanocytes being directly exposed to the water, — 

 a fact which I tliiuk is nearly, if not quite, admitted also by 

 F. E. ScHULZE ('99rt, p. 209 ; ig'a, p. 98), in that the existence 

 of a basement membrane at the place is denied by him and that 

 of a pavement epithelium held doubtful, while the choanocyte 

 layer is considered as resting on a relatively wide-meshed net- 

 iforh of trabecuhr. 



Be that as it may, F. E. Schulze's representation of certain 

 nuclei and cells as pinacocj^tes seems to be open to discussion. In 

 E. uftpergUlum ('80, pp. 669-671 ; '87, pp. 23, 24), he distinguished 

 the three following kinds of nuclei or cells in the trabecuhe : 



1) Small, spherical nuclei, abundantly and tolerably uniformly 

 scattered. These were seen on profile view to project a little 

 above the general surface of the trabecuhe, and were thus con- 

 sidered to occupy the most superficial position and on that ac- 

 count to represent flat epithelial cells, whose outlines were 

 certainly not seen. The nuclei in question evidently corresponds — 

 in part at least — to those which 1 have called the trabecular 

 nuclei. It is then important to decide if their seat in the 

 trabeculte is really superficial in relation to that of certain other 

 nuclei or cells in the same. This is by no means so, to judge 



