DR. ARTHUR FARRE ON THE ORGAN OF HEARING IN CRUSTACEA. 
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surrounding the sac, and thus increasing the analogy between this and the portio 
dura and mollis of the seventh pair of nerves in the higher Vertebrata. 
Next, the remarkable arrangement of ciliated processes immediately overlying this 
plexus, with each process filled with nerve granules, exhibits an apparatus for extend- 
ing the extremities of the nerves in such a manner as to render them sensitive to the 
most delicate vibration of the fluid with which the sac is filled. But to heighten the 
effect of this the grains of sand are added, thus forming adventitious otolithes, which, 
moving freely in the fluid contents of the sac, would considerably increase the vibra- 
tion of that fluid. 
But it is probable that the nerves are also more powerfully affected by the imme- 
diate contact of the stony particles themselves, since if they were only added for the 
purpose of multiplying the vibration, these would still have been rendered appre- 
ciable by the simple expansion of the plexus around the sac, without the necessity for 
a more complex apparatus. But the fact that the ciliated processes are always 
arranged upon that part of the surface of the sac which in the usual position of the 
animal would be lowest, so that the stones would by gravitation be constantly in 
contact with or near to them, seems to point to the immediate contact of these two 
parts as a condition essential to the performance of the functions of the organ. Thus 
the least vibration in the fluid would throw one or more of the particles into contact 
with one or more ciliated processes. And in consideration of the number of these 
particles and the readiness with which they would move in the fluid, and further of the 
extent of the ciliated surface and the delicate and abundant pubescence with which 
each process is clothed, it would seem hardly possible that the slightest vibration 
could occur in that fluid without throwing a particle of sand into contact with one of 
the processes, and thus causing the vibration to be conveyed to the nerve at its base. 
Now the mode in which vibration may be excited in the fluid is two-fold. The 
membrane expanded upon the upper surface of the joint containing the sac, would 
receive the vibrations and transmit them to the sac through the medium of the inter- 
vening flesh, which would thus supply the place of a peri-lymph. And again, they 
would be transmitted from the parietes of the sac to the fluid contained within, 
while in those species in which there is no fenestra and no membrane, the vibra- 
tions could only be communicated through the general surface of the parietes; as 
is the case for example in Cephalopods and fishes which have no external membrane. 
But further, the seat of this organ being a portion of the antenna, seems to con- 
nect the exercise of its function in some degree at least with the office of the antenna ; 
for it is obvious that the latter could not be brought into play without causing in 
the fluid contents of the sac, an agitation similar to that which would be produced by 
the undulations of the surrounding fluid striking upon the membrane, or the parietes 
of the antenna, and exciting corresponding undulations within. 
In this view of the matter, it would seem that the attributes of this organ are 
capable of being called forth by the exercise of the same mechanism as that which is 
furnished for the purpose of supplying the animal with its most delicate sense of 
