566 
H. V. WILSON 
covered with a peculiar layer, cuticular in appearance, probably 
formed by fusion of epithelial cells that undergo a gelatinizing 
metamorphosis. 
The case of Reniera. In Reniera aquaeductus Metschnikoff 
(76) thought that he was able with silver nitrate to demonstrate 
clearly cell contours in the epidermis. Keller a little later (78) 
studied the histology of Reniera and found that the silver method 
did give him a well marked system of dark lines forming poly- 
gonal meshes. But in many meshes no nuclei were present, while 
in others they lay in the extreme corner of the mesh, or again, 
they often lay directly upon the lines. For these and other reasons 
Keller believed that the meshes did not represent epithelial cells 
but were to be looked on as artefacts. I agree with Keller in this 
interpretation. Keller watched the closing of pores in living 
preparations under the microscope, but does not mention any 
structure such as the pore membranes of this paper. He is per- 
fectly right in discrediting the idea that the pores are closed by the 
contraction of surrounding muscle-hke cells, and is substantially 
in the right in maintaining that the pores open and close through 
the movements (contraction) of a superficial layer. As to the 
nature of this layer Keller at that time followed Haeckel and 
believed that the outer part of the sponge body (what is usually 
called epidermis or ectoderm plus mesenchyme or mesoderm) is 
composed of a soft living sarcode (exoderm of Haeckel) in which 
certain structures, spicules and some cells, are imbedded. This 
conception became untenable with the publication of F. E. 
Schulze's ''Untersuchungen" (1875-81). 
Nature of pores. In the Calcarea specialized pore cells, poro- 
cytes, are described, the pore being a perforation of such a cell 
and therefore intracellular. The porocytes lie at the surface 
of the simpler olynthus-like forms (Clathrina), but in the 
more complex Heterocoela they are said to occupy a position in 
"the walls of the flagellated chambers. The apertures at the sur- 
face of the Heterocoela, dermal pores, are usually thought of as 
intercellular gaps, i.e., as apertures each of which is bounded by 
numerous cells of the epidermis (Minchin '00, pp. 27, 48). In the 
Monaxonidaand other Demospongiae we find as in the Heterocoela, 
