CHAPTER VI 



Sensory Discrimination: Hearing 



§ 31. Hearing in lower invertebrates 



The sense of hearing, in all air-dwelling animals, is that 

 sense whose adequate stimulus consists in air vibrations; 

 for human beings these vibrations may reach a frequency of 

 50,000 (single vibrations) in one second and still produce an 

 auditory sensation. But the meaning of the term "hear- 

 ing" for water-dwelhng animals, and hence for most of the 

 lowest forms of animal life, is more difficult to determine. 



■^Tin the Protozoa it seems to have no meaning at all; the 

 reactions of these animals to water vibrations are indistin- 

 guishable from their reactions to mechanical stimulation. 

 But in some of the coelenterates the possibiHty of a specific 

 auditory sensation quality has been suggested by the dis- 

 covery of a pecuhar sense organ. While varying in its 

 structure in different genera and orders of ccelenterate 

 animals, this organ consists typically of a small sac, filled 



,^ with fluid and containing one or more mineral bodies, 

 ^^pa^ently these latter could operate in connection with a 

 stimulus"T9nly when the stimulus was constituted by shaking 

 the animal, or>^ some way disturbing its equihbrium. 

 They might then sbs!£^as means for the reception of water 

 vibrations, as the ear serves for the reception of air vibra- 

 tions ; they might, in short, be primitive organs of hearing. 

 Accordingly the term "otocysts" was given to organs 

 of this t3^e wherever they were found in the animal 



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