DR. ARTHUR FARRE ON THE ORGAN OF HEARING IN CRUSTACEA. 
235 
somewhat like an auricle, is more convex in its outer than its inner margin, and ter- 
minates below in a slight appendix. It is of a delicate horny structure, of the con- 
sistence of thin quill, and is nearly transparent, so as to admit of its contents being 
seen through the parietes. 
On removing a portion of the upper surface so as freely to expose the cavity, a 
number of minute particles of siliceous sand are found lying at the bottom, PI. IX. 
figs. 5 and 6. In the first specimens which I examined, the presence of these appeared 
to be accidental ; but as on pushing the inquiry further I invariably found them, 
not only in this species, but in every other in which the organ existed, I began to 
regard them as constituting essential parts of the structure ; as in fact, subsidiary 
otolithes derived from without, and supplying the place of these calcareous bodies 
which are found in various classes as a permanent portion of the auditory apparatus. 
These particles, no doubt, gain admission by the aperture just noticed, and as they 
are of nearly uniform size, or at least not exceeding a certain size, the largest being 
about TTToth of inch in diameter, the slight valve at the aperture may be regarded as 
serving the purpose of a regulator, while the water would be allowed to flow freely 
at all times by the circular opening. The margin of this aperture is surrounded in- 
ternally by a chevaux de frise of hairs, PI. IX. fig. 7- «• These are pointed forwards, 
and may be there placed either for the purpose of preventing the ingress of soft or 
flocculent particles with which the cavity might become clogged, or for regulating the 
admission of the particles of sand into the cavity. 
Along the lower surface of the vestibular sac is seen running a semicircular line, 
broader at its upper than its lower extremity. This part is more easily examined 
after the sand has been washed away by agitation under water, PI. IX. figs. 6 and 7- b. 
It is then seen, with a power of 18-linear, to consist of several rows of ciliated pro- 
cesses, of which one row is more regular and prominent than the rest, and crests the 
entire margin of the ridge. The processes diminish in size and number on either 
side, and are in some places seen in groups, but always assume the general form re- 
presented in fig. 7- 
With a power of from 100 to 200 linear these processes are seen to be hollow, and 
to be covered with a fine down of hairs of exquisite delicacy, while in their interior 
are contained numerous minute granules which are apparently nerve-granules, PI. IX. 
fig. 8. These processes are inflated at their base, so as to form a globular swelling, 
where they are articulated to corresponding circular apertures in the walls of the sac 
from which they spring. 
Immediately beneath this crescentic arrangement of processes lies the plexus of 
the auditory nerve, filling up a slight groove upon the under surface of the sac. The 
auditory nerve has a separate and distinct origin from the supra-cesophageal ganglion. 
It arises by two delicate branches between the lesser and greater antennai nerves, 
and proceeds directly upwards and outwards to reach the under surface of the vesti- 
bular sac, PI. IX. fig. 10. c c. Here it expands into a plexus, covering the whole 
2 i 
MDCCCXLIII. 
