IN THE DECAPOD CRUSTACEANS. 33 
The hairs to which M. Hensen gives the name of “ auditory 
hairs” are of three kinds. 1. Hairs with otolites. 2. Free 
hairs of the auditory sacs. 3. Auditory hairs of the outer 
surface of the body. 
The hairs belonging to the first class are those which are 
in contact with the otolites in the auditory sacs. These 
hairs are generally curved, and their point is inserted be- 
tween the otolites, as is the case in the crab, lobster, and 
shrimp, or they may even penetrate into the interior of an 
otolite, as may be seen in the auditory sacs in the tail of 
Mysis. The otolites are often held in suspension by these hairs. 
It is impossible, in reading this description, not to recall the 
sete described by Max Schultze and Frank-Hilhardt Schultze 
as existing in the auditory organ of fishes, and which sete 
are also said to have their points in contact with the otolites. 
M. Hensen notices analogous arrangements (vibratilecilia, 
&e.), which are observed in the auditory sacculi of other 
animals. He even thinks that he has noticed in some cases 
(Pisidium among the Mollusca, Cydippe among the Cilio- 
grada) setee penetrating into the otolites, as in the Crus- 
tacea. 
The crabs, and the Brachyura in general, are entirely 
without otolites; but their auditory sacs are nevertheless 
furnished with hairs. In Carcinas menas M. Hensen has 
counted about 800 hairs immersed in the fluid of the audi- 
tory sac. This absence of otolites in the crabs is the less 
surprising when we find that in the Lobster the auditory 
sae contains an entire row of hairs, which in their conforma- 
tion precisely resemble the otolitic hairs, but which never come 
in contact with those bodies. Some auditory hairs, there- 
fore, may be thrown into vibration without the mediation of 
otolites. These hairs sometimes existing in a space which is 
only partially closed (open auditory sacs), it is rendered 
probable that they may be equally capable of fulfillmg their 
functions if they were placed on the surface of the body. 
This consideration has led M. Hensen to regard as acoustic 
organs, certain hairs on the surface of the Crustacea which 
present the same structure as the hairs connected with 
otolites. 
These external auditory hairs have been studied by M. 
Hensen in the Carides. They are especially numerous in 
those species, or in young individuals (larvee), which have no 
internal auditory organs. In the Carides, these hairs are 
situated on the upper surface of the basal portion of the 
inner antenna, and on the second joint of the outer pair. 
They are found also on the tail, a circumstance at which we 
VOL. V.— NEW SER. c 
