858 THE AMERICAN NATURALIST. (Vor. XXXV. 
has been made by C. W. Prentiss.' In Palzemonetes, as in most 
other macrurans, the otocyst is a sac lodged in the basal segment 
of the antennule and opening dorsally by a constricted aperture 
partly covered by a scale-like fold. The sac is lined with cuticula 
which at the aperture is continuous with the animal's external shell. 
On an elevation rising from the floor of the sac is a horseshoe- 
shaped double row of from forty-five to fifty-eight hairs. The hairs 
are plumed and instead of being straight, as the tactile hairs of the 
outer surface are, they are bent so that the distal part of each shaft 
makes an angle of 120 degrees with its shorter base. Each hair is 
attached to the sac by a thin-walled chitinous bulb, thus allowing the 
hair as a whole to move freely. In the tangle formed by the cross- 
ing of the hairs are lodged fine grains of sand and organic detritus 
constituting an otolith. Every hair has at its base a group of matrix 
cells by which its chitinous wall was secreted. A single nerve fibre 
leads from the base of the hair inward to the brain, where it termi- 
nates in many fine branches. Each fibre has on its course a single 
cell body, so that each “auditory” hair is innervated by a single 
neurone. The same is true of the tactile hairs of the general surface, 
but the olfactory hairs, on the contrary, are innervated each by as 
many as a hundred neurones. Not only do olfactory and tactile 
hairs differ in this respect, but they can also be distinguished by the 
fact that in the olfactory hair the nerve fibres pass far out throug 
the axis of the hair towards its tip, but in the tactile hair the single 
fibre ends at the base of the hair. An otocyst essentially similar to 
that in Palamonetes was found in Crangon and in Cambarus. 
In the common green crab, Carcinus, the otocyst is closed, contains 
no otolith, and its hairs are arranged in three groups instead of one. 
The innervation of these hairs is the same as in the macrurans 
studied. 
Every time a shrimp or crab casts its shell, the cuticular lining of 
the otocyst, the attached hairs, and the otolith, if such be present, 
are discharged. As a preparatory step to this change, the matrix 
cells form a new hair under each old one, the new hair being half 
inverted in that the tip is pushed back into the base as the end of 
a finger of a glove might be infolded into the rest of the finger. 
When the skin is shed the new hairs are in part drawn out by the 
retreating skin to which they are slightly attached and in part 
1 Prentiss, C. W. The Otocyst of Decapod Crustacea: its Structure, Devel 
opment, and Function, Bull. Mus. Comp. Zoil., vol. xxxvi (1901), pp. 167-75" 
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