II. 

 Biological Theories. 



IV.— SUPPOSED AUDITORY ORGANS. 



''PHREE groups of phenomena appear to have been imperfectly 

 1 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 {e.g., Mysis), as auditory sacs 

 in lobsters, crayfishes, etc., and as auditory hairs or setse 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 it in such a position that he could' 

 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." " Tliat these plumose hairs then really serve for hearing may be 

 inferred from the observed fact that they respond to sound- 

 vibrations.'" " 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 liave 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,. 



