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-dwelling animals, and hence for most of the 

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

 In 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 ccelenterates the possibility of a specific 

 auditory sensation quality has been suggested by the dis- 

 covery of a peculiar 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. 

 Apparently these latter could operate in connection with a 

 stimulus only when the stimulus was constituted by shaking 

 the animal, or in some way disturbing its equilibrium. 

 They might then serve 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 type wherever they were found in the animal 



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