CRUSTACEA. 
277 
if these crustaeea hear a sound, they suddenly sink to the 
bottom, where they remain quiet for a time. Mysis spinulosus 
is very excitjible, and all that is said of Paldamon can be said of 
Mysis in a higher degree. 
The organs of hearing of Mysis being in the posterior pair of 
pleopoda, it is very easy to remove the two inner branches of 
that appendage, and with them all the auditory apparatus of 
the tail. Under this condition the author could not get any of 
the animals so treated to live beyond the 26-28 hours, and 
these for a short time before death generally show symptoms 
of excitability. The cause of this excitability was difficult to 
establish, since the animals were not in a normal condition; 
but several very convincing experiments induced the author to 
believe that sound was the true cause. 
Dr. Uensen had fully anticipated that the hearing-organs, 
which had been removed, were essentially necessary for the 
effect, and therefore was a little disappointed at the result. 
He has observed Carcinus masnas to stop upon the least noise ; 
if walking about a room at night, it will immediately stop if * 
called to. He attempted to blind the animals ; but they so soon 
died iti consequence, that no conclusions could be arrived at. 
In many of these experiments there must be very considerable 
doubt as to the result, in consequence of the animals being so 
very susceptible to the slightest movement of air that came 
into contact with them. 
Having arrived by these experiments at the conclusion that 
these Decapod Crustacea were conscious of sound, his next 
object is to show through what organs the consciousness of 
sound was conveyed to them. 
Although for some time Dr. Hensen felt convinced that cer- 
tain hairs had some connexion with the consciousness of sound, 
yet it was only after a long study of this interesting problem 
that it became clear to him that each hair was adapted to 
receive its own peculiar sound. He thinks that each hair vibrates 
to a separate note ; but of this he has not any strong evidence, 
beyond the circumstance that the auditory hairs differ from 
each other in length as well as in diameter. 
Experiments on the auditory hairs on the tail of Mysis were 
made in the following manner : — A small brass box with a glass 
bottom, a hole through which was closed by a cork, into which 
a needle, that held the animal to be experimented upon, was 
thrust. The box was filled with water, and thus the animal, with 
the aid of a good penetrating lens, could be observed in every 
position. On the opposite side of 'the box is firmly screwed a 
sound- guiding apparatus, which resembles somewhat the cavum 
tympani, and is brought in elose eontaet with the animal, while 
at the opposite extremity of the tube is a plate on which a piece 
of india-rubber is scr(5wcd ; this plate divides the inner pipe 
