membranous visceral wall.

A delicate, but broad, triangular membranous process, about 1/4th 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.

In fact this cavity may be said to be co-extensive with the attached
part of the mantle,--the viscera, enclosed within their delicate
"peritoneal" membranous coat, projecting into and nearly filling it, but
nevertheless leaving a clear space between themselves and the delicate
posterior wall of the mantle.

A layer of the "peritoneal" membrane extends from the posterior edge of
the muscular expansion which lies between the shell-muscles and from the
upper wall of the dilatation of the vena cava, and passes upwards and
backwards like a diaphragm to the under surfaces of the gizzard and
liver. It is traversed by the aorta, to whose coats it closely adheres.

Along a line nearly corresponding with the horny band which proceeds
from the insertions of the shell-muscles and encircles the mantle below,
the pallial wall is produced inwards and forwards into a membranous fold
or ligament, which I will call the pallio-visceral ligament; and this
pallio-visceral ligament becoming attached to various viscera, divides
the great fifth chamber into an anterior inferior, and a posterior
superior portion, which communicate freely with one another.

Commencing with its extreme right-hand end, the ligament is inserted
into the line of reflection of the mantle, and then into the wall of the
oviduct, which becomes enclosed as it were within the ligament. The
latter then ends in a free edge on the inner side of the oviduct, and is
continued along it until it reaches the inferior surface of the apex of
the ovary, into which it is inserted.

The free edge is arcuated; and the rectum passes over it, but is in no
way connected with it.

Here, therefore, is one great passage of communication between the
anterior and posterior divisions of the fifth chamber.

On the left side, this aperture is limited by the heart, whose posterior
edge is, on the left side, connected by means of a ligamentous