lle 
Biological Theories. 
IV.—SUPPOSED AUDITORY ORGANS. 
HREE groups of phenomena appear to have been imperfectly 
distinguished in the minds of those who have ascribed auditory 
functions to the organs known as tentaculocysts in jelly-fishes, as. 
otocysts in mollusca and some crustacea (¢.g., Mysis), as auditory sacs. 
in lobsters, crayfishes, etc., and as auditory hairs or sete in the same: 
and other crustaceans. 
The groups of phenomena are those associated with the trans- 
mission of sound-waves in air, of sound-waves in water, and of waves 
of other kinds in water. The nature of the confusion will be seen by 
a consideration of the following quotation from Sir John Lubbock’s. 
«« Senses of Animals ’”’ :— 
‘‘ Hensen took a Mysis, and fixed itin sucha position that hecould 
watch particular hairs with a microscope. He then sounded a scale: 
to most of the notes the hair remained entirely passive, but to some 
one it responded so violently and vibrated so rapidly as to become: 
invisible.” <‘‘ That these plumose hairs then really serve for hearing may be 
inferved . . . from the observed fact that they vespond to sound- 
vibyations.” ‘* Hensen’s observations have been repeated and verified. 
by Helmholtz.” (Op. cit., pp. 93, 94). 
The portions of the passage which I have omitted refer to the 
existence of nerves in relation with these hairs. 
Two criticisms are at once suggested. In the first place, the 
inference is unsound. Because the human eyelashes ‘respond to: 
sound-vibrations,”’ it is not safe to infer that they ‘‘ really serve for 
hearing,’ and it is not any more safe to infer from the ‘“‘ observed fact” 
that certain plumose hairs in Mysis ‘‘ respond to sound-vibrations ” 
that these serve for hearing. 
In the second place, there appears to have been a misapprehension: 
as to what constitutes a sound-wave in water. 
If a sounding tuning-fork be pressed against the side of an 
aquarium, two kinds of waves are produced in the water, surface- 
waves and sound-waves. The surface-waves are visible to the naked 
eye and travel slowly across the surface, that is, sufficiently slowly to 
be followed by the eye. Each of these is a movement of the water, 
