238 DR. ARTHUR FARRE ON THE ORGAN OF HEARING IN CRUSTACEA. 
of an osseous labyrinth with its fenestra ovalis and membrane, while, as in other 
primitive forms of the organ of hearing, no trace exists of either semicircular canals, 
tympanic cavity and ossicula, or external concha. 
There are, however, some features now described in respect of which the organ 
differs from any other form with which we are acquainted, viz. in the remarkable 
apparatus in which the auditory nerve terminates, and in the singular substitute for 
otolithes found so constantly in the vestibular sac. With regard to the termination 
of the acoustic nerve, I have shown that the plexus into which it divides closely sur- 
rounds the vestibular sac, but is most extensively developed immediately beneath the 
row of processes lining the cavity, and is lodged in a slight depression or groove from 
the crest or opposite surface of which they spring. It is easy to observe the row of 
apertures by which this part of the sac is pierced, each pore leading into the central 
cavity of a process, and each process filled with granules of apparently nerve matter, 
loosely contained in the interior of the process, and which escapes when it is detached 
or torn, Plate IX. fig. 8. The water, which is freely admitted into the vestibular sac 
by the aperture in its upper part, supplies the place of an ento-lymph, and constitutes 
perhaps the only example of an organ of hearing in which the same fluid by which 
the vibrations are communicated, is received directly into the chamber upon which 
the acoustic nerve is expanded. 
The grains of sand appear to supply the place of otolithes ; and in reference to this; 
which is not the least remarkable feature in the construction of the organ, it may be 
observed that the grains appear to consist almost entirely of particles of siliceous 
sand, and are certainly not cretaceous bodies secreted by the organ itself. They are 
transparent and angular, and are unaffected by acids. I have observed also that their 
size is not greater than would allow of their entering by the valvular aperture; and 
that in those species where the aperture is large and free, as in Palinurus, the grains 
are coarse in proportion, while in the other species, where the valve is closer, they are 
correspondingly fine. The circumstance of a natural structure being supplied by 
artificial means is not without its parallel in the animal kingdom, and can hardly fail 
to suggest the familiar example of the stomach of granivorous birds, into which stones 
are taken for the purpose of supplying the office of gastric teeth, and become essential 
to the due performance of the function of that organ. 
Such being the nature of the apparatus, little of explanation appears to be required 
with regard to the function of its several parts. 
The fact of the delicate nerve having a separate origin from the supra-oesophageai 
ganglion, and being distributed in the form of a plexus around the sac, seems to pro- 
claim this a nerve of special sense, more particularly as the lesser antennal nerve 
passes so close to the sac in its course through the antenna (Plate IX. fig. 10. b.) that 
for ordinary purposes the sac might have been most readily supplied from it. To 
this sac, however, the antennal nerve sends off only one or two delicate twigs, and 
those apparently for the purpose of supplying the tegument or muscles immediately 
