82 THE ANATOMY OF THE NAUTILUS POMPILIUS 
The two apertures lie just above the edge of the fold of membrane 
which runs from the inner root of the larger or outer branchia, across 
the branchial cavity and beneath the rectum, to the other side. 
These apertures lead into five sacs, which collectively constitute 
what has been described as the pericardium. The sacs into which 
the superior apertures open, by a short wide canal with folded walls, 
are situated on each side of and above the rectum. Their inner 
boundaries are separated by a space of not less than ths of an inch 
in width, in which lie the vena cava and the oviduct. Each cavity 
has a rounded circumference, and a transverse diameter of about 
half an inch. In a direction at right angles to this diameter the 
dimensions vary with its state of distention; but a quarter of an inch 
would be a fair average. 
The anterior or outer wall of the cavity is formed by the mantle ; 
the posterior, inner, or visceral wall by a delicate membrane. The 
former separates it from the branchial cavity; the latter from the 
fifth sac, to be described by-and-by. I could find no natural aperture 
in the thin inner wall, so that I conceive no communication can take 
place between either of these sacs and the fifth sac. 
Two irregular, flattened, brownish, soft plates depend from the 
posterior wall of the sac into its cavity; their attached edges are 
fixed along a line which is directed from behind obliquely forwards 
and upwards. 
The outer and smaller of the inferior apertures on each side leads 
into a sac of similar dimensions and constitution to the preceding, but 
having a less rounded outline in consequence of its being flattened in 
one direction against its fellow of the opposite side, from which it 
is separated only by a delicate membranous wall, whilst on another 
side it is applied against the inferior wall of the superior sac, and is 
in like manner separated from it only by a thin and membranous 
partition. 
Like the upper sacs, each of these has two dark-brown, lamellar, 
glandular masses depending from its membranous visceral wall. 
A delicate, but broad, triangular membranous process, about jth 
of an inch long, hangs down freely from the visceral wall of the cavity 
just behind the opening of the short canal which connects the sac 
with its aperture. 
The third and largest aperture on each side opens directly into a 
very large fifth cavity, whose boundary is formed anteriorly by the 
visceral walls of the sacs already described, and behind this by the 
mantle itself as far as the horny band which marks and connects 
the insertion of the shell-muscles. 
