MODE OF HEARING. 



93 



a calcareous matter embedded in an organic substance. 

 That Crustacea do, as a matter of fact, possess the power 

 of perceiving sounds, there can be no doubt. Hensen 

 himself has made various experiments on the subject. 

 Moreover, strychnine possesses the peculiar property of 

 augmenting the reflex power of the nervous centres. 

 Taking advantage of this, Hensen placed some shrimps 

 in sea-water containing strychnine. He then found 

 ihat they became ex- 

 tremely sensitive to even 

 very slight noises. Further 

 than this, Hensen availed 

 himself of Helmholtz's re- 

 searches on the perception 

 of sound, and, suspecting 

 that the different hairs 

 might be affected by dif- 

 erent notes, found that was 

 actually the case. 



The vibration of the hairs 

 is mechanical, not depend- 

 ing on the life of the 

 animal. 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. When the 

 note ceased, the hair became quiet ; as soon as it was 

 resounded, the hair at once began to vibrate again. 

 Other hairs in the same way responded to other notes. 

 The relation of the hairs to particular notes is probably 

 determined by various conditions ; for instance, by its 

 length, thickness, etc. 



Fig. 63. — Tail of yfj/ais vulgaris, show- 

 ing the auditory organ. 



