540 
H. y. WILSON 
uous with the canal hning in general. Round most of the epider- 
mal nuclei the protoplasm is aggregated, forming thickened more 
deeply staining areas which shade ofT into the internuclear portion. 
The structures marked p.m., which I propose to call pore mem- 
branes, and which so far as I know have not been described, are 
extensions of the epidermal sheet over the pores. These exten- 
sions are so thin that they afford an especially favorable oppor- 
tunity for studying the intimate structure of the epidermal layer. 
Their nature is learned when the process of pore closure is 
studied, and it will be well now to give a description of this 
process. 
The dermal pores of the monaxonid sponges are customarily 
referred to as mere perforations of the dermal membrane. They 
are in reality short canals leading from the exterior into the sub- 
dermal chambers. Ordinarily the dermal membrane, while thin, 
is of such thickness that the actual aperture at the surface of the 
sponge is distinguishable, on focussing, from the canal itself. 
Thus in fig. 1, the pore membrane, p.vi., partially closes the aper- 
ture and is distinctly seen when the epidermis is in focus. On 
focussing a little lower the wall of the canal itself, p.c, comes into 
view as a distinct line which often exhibits a nucleated thickening 
or two. The nucleated thickening may as in the case of two of 
the pores shown in fig. 1, extend out into the mesenchyme in the 
shape of a slender process. I propose to restrict the use of the 
term pore (viz., dermal pore), to the actual aperture, and to desig- 
nate the short canal as the pore canal. 
When the pores are widely open as is the case with pore 1, in 
fig. 1, there is no sign of a pore membrane. The epidermis is 
directly continuous at the edge of the pore with the lining of the 
pore canal. But even when the sponges are fixed at once on being 
taken from the live box, some of the pores will be partially closed 
and will show the pore membrane, p.m. This thin extension of 
the epidermal sheet in sponges so preserved will usually be 
found barely extending beyond the margin of the pore and it may 
or may not include a nucleus. It is a single thin layer which yet 
is continuous with both epidermis and the lining of the pore canal. 
Since it has the structure of the epidermis I speak of it and regard 
