34 HENSEN, ON THE AUDITORY ORGAN 
need not be too much surprised when we remember the 
caudal ears of the Opossum-shrimp. 
The physiological interpretation given by M. Hensen of 
these hairs on the external surface is calculated, at first 
sight, to excite some mistrust. It should be stated, there- 
fore, that these organs present certain peculiarities of struc- 
ture which distinguish them from the other hairs of the sur- 
face, and cause them to resemble the hairs in the acoustic 
sacs. The characters common to all the auditory hairs of 
M. Hensen may be thus summed up:—1. They are always 
implanted over a hole in the chitmous membrane. 2. The 
border of this hole is elevated on one side into a sort of 
tooth. 38. The stem of the hair does not rest directly upon 
tlie hole, but is supported by a very delicate membrane, 
which is often dilated so as to form a sort of ampulla at the base 
of the hair. This arrangement is calculated to isolate the 
hair from any agitation proceeding from the interior of the 
body, and, at the same time, perhaps to facilitate its vibra- 
tions. 4. Lastly, the stem of each auditory hair presents a 
sort of appendage (the languette), to which the nerve of the 
hair is attached. Properly speaking, the nerve terminates 
in a ganglion-cell before it reaches the hair; but from this 
cell arises a minute and slender nerve-cord (Chorda, Hens.), 
which enters the canal of the hair, and is attached to the 
“languette.’”’ The other hairs of the surface do not exhibit 
the same complex structure, and, in particular, are wholly 
devoid of any nervous cord. 
The auditory hairs are of course, renewed at each moult. 
The new hairs, however, are not contained within the old 
ones, as is commonly said, but are formed beneath the chi- 
tinous skeleton. In comparing the new integument of the 
animal to a glove, the hairs would be placed like the fingers 
of the glove, if we were to suppose them withdrawn into the in- 
side of the glove in sucha way that merely their pomts were 
visible at the surface. This observation, however, has not 
escaped the notice of Mr. Spence Bate and of Leydig. At 
the moment of throwing off the old sheil the cast-off shell 
drags with it the extremities of the new hairs, and draws 
out the invaginated portion. The point up to which the 
extremity of the new hair penetrates into the old hair coin- 
cides with the point of attachment of the nervous cord, that 
is to say, with the border of the languette. It should be 
stated, also, that shortly before the ecdysis a second nervous 
cord appears alongside the former one, and that the latter is 
thrown off at the moment of ecdysis. Moreover, it appears 
to change its nature, for it becomes, as it were, chitinized at 
the instant it is thrown off. These auditory hairs, 1t may 
