480 E. A. MINOHIN. 
from above. Fig. 4 represents the whole of a small colony with 
three oscula, magnified five times linear. It will be seen that 
the small colony forms a more or less flat plexus of narrow 
tubes, from which chimney-like oscular tubes arise of about 
0:325 mm.in diameter. In fact, it grows in precisely the same 
manner as the specimens of Leucosolenia coriacea at 
Plymouth, in which I observed and described a sieve mem- 
brane over the oscular opening. 
If now we study the edge of one of the larger oscula with a 
lens by transmitted light, we see that it has a margin of about 
half a millimeter in width where the wall is more transparent 
than in other parts of the oscular tube, and a very short dis- 
tance above where this more transparent part begins an 
opaque line can be seen running round the whole osculum 
(fig. 2 a, s.). 
This clearer margin indicates the line at which the collared 
endoderm stops short, so that the clearer margin is lined with 
ectoderm within and without, while the more opaque part of 
the oscula tube is lined by collar-cells on the inner side. The 
opaque line s is a muscular sphincter, by the contraction of 
which the osculum can be closed, and occurring alike in the 
larger and in the smaller oscula. 
To proceed now to the study of sections and preparations. 
Fig. 6 a, 6, c, d, represent four sections from a ‘continuous 
series through one of the oscula of the sponge represented in 
fig. 4, after hardening in 4 per cent. osmic acid, soaking for 
an hour (on the slide) in picro-carmine, in order to counteract 
the blackening of the osmic acid, and finally staining with 
hematoxylin. a, b,and ¢ are three sections about the middle 
of the series, 6 being the next section after a, and c the next but 
one after 6; while d, in which the osculum is cut tangentially, 
is the thirteenth section after a. In all the sections the spicules 
have been carried by the edge of the razor towards the left, 
injuring the ectoderm a little. 
Fig. 6 a shows well the general structure of the oscular 
wall. The collar-cells stop short suddenly at a point, and it 
can be seen plainly in figs. 6 a and ¢ that they are continued 
