286 MR. J. D. MACDONALD ON THE ANATOMY OF NAUTILUS UMBILICATUS 
large oval opening in the anterior wall of the ovary, with a 
thickened puckered margin, at which the investing and lining 
membranes are continuous. The posterior wall of the pericar- 
dium, on the right side, acts as a valve to this opening. 
Although I have observed the albuminous fluid just alluded 
to free in the abdominal cavity of N. Pompilius, I did not dis- 
cover the orifice by which it must have escaped from the 
ovary. 
z. The narrow oviduct is seen passing forwards on the right side of 
the rectum, and ending in a thickened and apparently glandular 
extremity, from the left side of which a fold of the investing 
membrane arches over the rectum near the termination of the 
long visceral nerves, lying below and in front of the reflected 
layer of peritoneum, which prevents the admission of the sea- 
water with the blood bathing the cephalic ganglia, oesopha- 
gus, and buccal mass. The lateral attaehment of this layer is 
represented by faint lines on the inner surface of the shell- 
muscle, extending upwards and backwards to the apex of the 
left hepatic mass. 
PLATE XV. 
Fig. 1. Dissection exhibiting the auditory sac of the right side, inside in situ, the 
cephalic ganglia, neighbouring parts, &c. 
a. The eye, beneath which is seen the hollow process resembling a 
tentacular sheath. 
b. Supra-oesophageal nervous mass, cylindroid in form, and present- 
ing a ganglioniform enlargement at either end, from which the 
optic nerves are given off. The stout fibrous envelope is slit 
open, exposing the reversed loops of four nerves which pass 
onwards to the buccal mass. 
c. Posterior division of the suboesophageal ganglia, giving off from 
its convex border a considerable number of nerves to the shell- 
muscles, and, on either side of the median line, two principal 
visceral nerves. 
d. Portion of anterior division of suboesophageal ganglia, with the 
auditory sac lying on the outer surface of its pedicle, near the 
point where all the principal cephalic nervous masses of the 
corresponding side unite. A small depression may be noticed 
on the upper and inner border of the cephalic cartilage in which 
the auditory capsule rests. 
