IN THE DECAPOD CRUSTACEANS. 85 
be further observed, are so complex in structure that it 
would be astonishing to find them formed like the scales on 
a butterfly’s wing by the metamorphosis of a single cell 
(Semper). In reality, a great many cells concur in their 
formation. 
However careful his anatomical researches, M. Hensen’s 
physiological conclusions drawn from them would appear 
very hazardous, had he not been successful in adding to their 
value by delicate experiment. It was first requisite to show 
that the Crustacea possess a delicate sense of hearing. This 
point M. Hensen has clearly demonstrated. The most con- 
clusive experiment consists in the placing of the prawns or 
opossum-shrimps in a vessel of sea-water containing some 
strychnine, an alkaloid which possesses the property. of 
augmenting the refiex power of the nervous centres. Under 
the influence of this agent, the Crustacean responds, by the 
most vigorous leaps, to the slightest noises which are heard 
in the house. Even when the creature has almost suc- 
cumbed to the effect of the poison to such an extent as to 
allow itself to be drawn about without resistance in the 
vessel, by the outer antenna, the slightest sound is sufficient to 
make it tear itself out of the forceps, and to fall to the 
bottom, a prey to tonic convulsions. 
It is evident, therefore, that the Crustacea are sensible to 
sounds. The second point was to determine whether the 
perception took place by means of the auditory hairs. 
Guided by the beautiful researches of Helmholtz on the 
perception of sounds, M. Hensen supposed that each audi- 
tory hair is capable of being thrown into vibration, exclu- 
sively of the rest, by a determinate note. Experiment con- 
firmed this hypothesis. If the attention be closely fixed 
upon the point of attachment of the chorda to the languette 
of a hair whilst the different notes of the gamut are sounded 
on a instrument, the point will become indistinct, or the 
entire hair may be thrown into powerful vibrations by certain 
notes. At the same time the neighbouring hairs generally 
remain perfectly still, and can only be made to vibrate by 
other notes. ‘The conditions by which the vibration of a hair 
is produced by one note rather than by another are doubtless 
multiple: amongst them may be enumerated the length and 
thickness either of the hair or of the languette. 
he physiological portion of M. Hensen’s memoir is of 
eveat interest. It shows us not only that the Crustacea can 
perceive sounds, but also that sounds affect these animals 
through organs to which he has given the name of auditory 
hairs, and it is difficult, seeing the force of his demonstra- 
tions, not to accept this term. 
