PRENTISS: THE OTOCYST OF DECAPOD CRUSTACEA. 219 
ratios to those of the volume of organ pipes, we should have, ¢f the hairs 
responded to different sound vibrations, an auditory organ with a range 
of three octaves. 
To prove that his hypothesis was correct, sound waves were conducted, 
by a mechanical contrivance modelled after the middle ear of mammals, 
into the water of a vessel containing Mysis, the so-called auditory hairs 
of which were under observation by the microscope. When notes of 
a certain group were sounded on a musical instrument, a certain hair 
would vibrate and disappear from view. Others would also respond, but 
each to different sets of notes. 
Having proved that the different hairs responded to different sound 
waves, Hensen next determined that Crustacea would react to vibratory 
stimuli. A resonant bar of wood was floated in a vessel containing free- 
swimming individuals of the genera Mysis and Palemon. When the bar 
was struck, both forms responded by a strong leap away from the source 
of the sound. Palemon reacted even more strongly when rendered 
sensitive by gradual strychnine poisoning. 
Milne-Edwards (76), Jourdain (’80), Delage (’87), and many others 
have accepted the sense of audition in Crustacea as a fact. 
Garbini (’80, p. 192) uneritically remarks: “ Che i crostacei odano 
e indubitato; lo sanno anche i pescatori, i quali devono avvicinarsi loro 
in silenzio” (That crustacea hear is undoubted ; this the fishermen know 
well, who, when they capture them, approach in silence). 
Individuals of Palemonetes varians, which he kept in an aquarium, 
sprang backward at the slightest sound. 
Delage (’87) was the first to discover another function than that of 
audition for the otocyst. By cutting off or destroying the sacs, he 
proved that they functioned also as organs of orientation. Animals so 
operated upon (Mysis, Palemon, and Polybius among Crustacea) were 
unable to keep their normal upright position in swimming. Blinding 
intensified the effect, showing that sight aided in orientation. 
The otocyst may therefore, in his opinion, be compared to the sim- 
plest form of the vertebrate ear, — that found in Myxime, — where the 
semicircular canals and utriculus serve the purpose of orientation, the 
sacculus that of audition (to intensity of sound). In the otocyst of 
Crustacea both functions are performed, he believes, by the same 
organ. 
Verworn (’92) proved that the otocyst of Ctenophores served simply 
for orientation, not being sensitive to sounds. 
Bunting (’93) confirms the conclusions of Delage as to the function 
