EPITHELIOID MEMBRANES IN MONAXONID SPONGES 539 
sections are far better than thin ones. They afford many places 
where, owing to the transparency of the tissues underlying the 
epidermis, the latter can be well seen. The sections were given 
an after stain with congo red, or with Delafield's haematoxylin 
followed by congo, and were mounted in balsam. To obviate 
the possible ill effects of imbedding, strips of the epidermis were 
torn off with forceps from the alcoholic material, were stained 
in haemalum and congo red, and mounted in balsam. Most of 
the pieces obtained in this way are too thick for study, but occa- 
sionally very thin strips peel off. 
As is well known the pores of sponges close and open. Prepara- 
tions of the epidermis with the pores widely open were made from 
sponges that had been kept in a live box. In a liv^e box placed 
where the tidal current is good, the sponge is usually found with 
the oscula and pores fully open and the canals dilated. If the 
sponge so expanded be suddenly plunged in the fixativ^e, the pores 
will not have time to contract. JMore useful preparations are those 
in which the pores are closed or half-closed. Sponges that have 
been kept a short time in running aquaria are found to be in this 
condition. 
A part of the dermal membrane as seen in a thick tangential 
section is show^n in fig. 1. Some of the pores are completely open 
and others nearly so. The wall of a small subdermal cavity is 
indicated by the line s.c.iv., and into this cavity the pores open. 
Beyond s.c.w. we come to a thicker part of the body separating 
the subdermal space shown from neighboring ones. In this part 
a few especially conspicuous mesenchmye cells, ??i.c., appear. 
They come into view when the microscope is focussed just below 
the surface of the sponge. The canal w^all shows some of the lin- 
ing cells, ex., as seen in optical section. They also appear of 
course only at a focus below the surface of the sponge. Over the 
subdermal cavity the figure shows the epidermis in focus. It 
appears as a continuous thin protoplasmic sheet without cell 
boundaries and studded with nuclei, ep.n. On focussing below 
the epidermis the mesenchyme cells of the dermal membrane 
would come into view. Below the mesenchyme lies the inner 
covering of the dermal membrane, an epithelioid la3'^er contin- 
