CHAPTER VI 





 SENSORY DISCRIMINATION: HEARING 



33. 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 " hearing " 

 for water-dwelling animals, and hence for most of the lowest 

 forms of animal life, is more difficult to determine. Tn the 

 Protozoa it seems to have no meaning at all ; the reactions of 

 these animals to water vibrations are indistinguishable from 

 their reactions to mechanical stimulation. But in some of 

 the ccelenterates the possibility of a specific auditory sensa- 

 tion quality has been suggested by the discovery of a pecul- 

 iar sense organ. While varying in its structure in different 

 genera and orders of ccelenterate animals, this organ con- 

 sists 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 dis- 

 turbing its equilibrium. They might then serve as means for 

 the reception of water vibrations, as the ear serves for the 

 reception of air vibrations ; 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 kingdom, and the mineral bodies in the otocysts were 

 called otoliths. 



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