234 
DR, ARTHUR FARRE ON THE ORGAN OF HEARING IN CRUSTACEA. 
the mouth of this little sac, immediately beneath the external orifice, is of a slightly 
horny substance and dark brown colour. A bristle may be easily passed into the 
orifice, as in the preceding species, PL X. fig. ]/• 
I feel that any explanation that might be offered as to the use of this organ would 
be little better than conjecture, since there is nothing in the structure of the parts to 
lead to any definite conclusion ; but there are one or two points which may be 
noted as peculiar. 
The organ is situated not far from the mouth, and is directed downwards. It ap- 
pears to be supplied by a separate nerve sent off from the supra- (esophageal ganglion, 
and I have ascertained, by repeated experiments, that it is the most sensitive part of 
the body ; since, while the mechanical irritation of any other parts excited only a 
slight movement in the limbs of the animal, when out of water, and somewhat feeble, 
the touching of this part was immediately followed by a violent and almost spasmodic 
(lapping of the tail. These circumstances, together with the situation of the organ, 
appear to point it out as intended possibly for the purpose of testing the quality of 
the food ; as in fact an organ of smell, evidently endowed with an exquisite sensi- 
bility ; but in the absence of other analogies I would be understood as offering this 
rather as a conjecture, subject to correction by further examination and experiment. 
I may add, that I have not observed this organ in any other than the two species 
just noticed. 
But it is to the true organ of hearing that I would more particularly direct atten- 
tion. This remarkable organ is situated in the base or first joint of the lesser pair 
of antennse, the joint being slightly dilated at its base, Pi. IX. fig. 2. a. 
Its precise seat is indicated externally by a tough membrane, covering an oval 
aperture in the upper surface of this joint, PI. IX. fig. 2. b. The membrane appears 
to be a continuation of the same structure which forms the shell, but in which the 
earthy matter is wanting. 
Towards the inner and anterior margin of this membrane, there exists a small 
round aperture into which a bristle can be easily passed, PL IX. fig. 3. This orifice 
is capable of being dilated into a slit-like aperture, which in some specimens appears 
to be facilitated by the detachment of a small flap of the calcareous shell, and the in- 
terposition of a portion of membrane between it and the body of the joints, so as to 
constitute a small valve at this part, by which the aperture is capable of being en- 
larged (fig. 3. a.). 
On removing this oval membrane, together with a portion of the surrounding shell, 
the internal organ is brought into view, completely imbedded in the soft integument 
and muscular structure of the antennse. By careful dissection these parts are easily 
removed, and the organ is seen suspended in the centre of the joint, being free on all 
sides, and having a single attachment at the inner and anterior angle around the 
margin of the aperture just noticed, PI. IX. fig. 4. 
This organ, the vestibular sac, nearly fills the cavity of the joint. It is shaped 
