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XI. On the Organ of Hearing in Crustacea. By Arthur Farre, M.D., F.R.S. 
Received June 15, — Read June 15, 1843. 
Although the existence of an organ of hearing in the class Crustacea has not 
altogether escaped the observation of anatomists, yet the descriptions which have 
been hitherto given of that structure have stopped short at the point where the in- 
terest of the subject begins. For while some general analogies have been traced 
between its more prominent and obvious parts, and those of the organ of hearing in 
other classes, the essential features in this remarkable piece of mechanism, as deve- 
loped by the aid of the microscope, have been quite overlooked. It is my object to 
supply this deficiency in the following account of some dissections which were com- 
menced many years ago, when my attention was more particularly directed to com- 
parative anatomy. 
But first it is important to point out an error which has arisen from the confusion 
of two separate and distinct organs situated, the one at the base of the larger or 
second pair of antennse, and the other at the base of the smaller or first pair, to each 
of which the function of an organ of hearing has been assigned by different ana- 
tomists. 
With regard to the organ situated at the base of the great antennse, which is cer- 
tainly not the organ of hearing, I have little of observation to offer. This organ in 
the Lobster ( Astacus marinus), which affords a familiar example, is situated on the 
under surface of the base of the great antennse on either side (Plate I. fig. ] . a a.), 
and consists of a small and slightly conical papilla, abruptly truncated, and having 
stretched over it a membrane, in the centre of which is an aperture capable of ad- 
mitting a small bristle, the level of the membrane being somewhat sunk below the 
margin of the papilla, which forms a slightly elevated ring around it. On making 
a section of this part, nothing more is discovered than a narrow canal in the fleshy 
substance, leading perpendicularly from the external orifice, and terminating abruptly 
at the depth of two lines ; neither the canal nor the adjacent parts exhibit anything 
remarkable in their structure. A distinct nerve however is sent off to this organ 
from the supra-oesophageal ganglion, taking its origin immediately behind that of 
the greater antennal nerve, and pursuing its course long and slender until it arrives 
immediately beneath the papilla, where it becomes lost, Plate IX. fig. 10 . e e, fig. 1 1 . d. 
A similar organ exists in the river Cray-fish ( Astacus fluviatilis) , but in this in- 
stance instead of a narrow canal, the part swells out into a small membranous 
chamber, still however destitute of anything remarkable in its structure, except that 
