COMPARED WITH THAT OF NAUTILUS POMPILIUS. 
279 
Organ of Hearing . — The thorough Inv^estigation of the various systems and organs of 
an animal previously almost wholly unknown, with a single mutilated specimen at the 
disposal of the anatomist, is attended with difficulties which can only be surmounted 
by the accomplished dissector. An undertaking of this kind fell to the lot of Professor 
Owen when he entered upon the examination of the first recent Pearly Nautilus that 
reached the shores of England, and the beautiful monograph resulting from the right 
use of this single opportunity is a lasting memorial of the genius of its author. The 
organ of hearing seems to have been the only matter of any importance that escaped 
the scrutiny of the Professor, and although 1 have been fortunate enough, myself, to 
discover the auditory capsules in the recent N. iimbllicatus, I can easily conceive the 
difficulty of detecting them in a specimen long preserved in spirits, which renders the 
tissues opaque and ill-adapted for microscopic investigation. 
The acoustic capsules in my specimen were about one-twelfth of an inch in dia- 
meter, subspherical in form, and situated at the union of the supra with the snboeso- 
phageal ganglia, but more especially connected with the short pedicle of the anterior 
division of the latter (Plate XV. fig. 1 d). I have been induced to look for them in 
this locality, bearing in mind the condition of the organ of hearing in most Gastero- 
poda, and recognizing the close affinity of the Nautilus to this order. 
In every instance the supra-oesophageal ganglia occupy the cephalic region, 
but on account of the great length of the neck and body anterior to the visceral 
nucleus in Heteropoda, the suboesophageal nervous masses suffer the maximum 
amount of backward displacement, and the whole nervous system approximates 
the homogangliate type. In this extreme case the ear is still preserved in the neigh- 
bourhood of the eye, and the special centres of vision and audition are incorporated 
with the supra-oesophageal ganglia, from which both the auditory and optic nerves 
arise. 
The inner wall of each ear-sac in the Nautilus is somewhat flattened, lying in con- 
tact with the nervous matter; but its more convex external surface rests in a little 
depression on the upper and internal border of the cephalic cartilage. It is enve- 
loped in a kind of fibrous tissue, and filled with a cretaceous pulp consisting of 
minute elliptical otokonia, which, when under a high power, present a bright and 
strongly refracting point near each extremity. These particles vary much in size, 
and are sometimes curiously combined, so as to appear double, or assume the form 
of a star or cross, &c. 
This simple auditory apparatus may be readily exposed by making an incision 
externally in the deep groove between the funnel lobe and the basal part of the 
tentacular sheaths, immediately in front of the hollow subocular process. 
The cilia lining an auditory sac containing minute otokonia, are always much 
finer than those required to impart a rotatory motion to a single spherical otolithe. 
I have not observed them in the ear of the Nautilus, although I cannot for a moment 
doubt their existence. 
