THE SPECIAL SENSES 



233 



auditory cells, specialized nerve cells, which often bear 

 delicate vibratile hairs (Fig. 145). Auditory organs of this 

 general character are known among the polyps, the worms, 

 the crustaceans, and the mollusks. In the common cray- 

 fish the " ears " are situated in the basal segment of the 

 inner antennae or feelers (Fig. 146). They consist each of 

 a small sac filled with liquid in which 

 are suspended several grains of sand 

 or other hard bodies. The inner 



FIG. 145. Auditory organ of a mollusk. a, audi- 

 tory nerve ; b, outer wall of connective tissue ; 

 c, cells with auditory hairs ; d, otolith. After 

 LETDIG. 



FIG. 146. Antenna of 

 cray -fish, with audi- 

 tory sac at base. 

 After HUXLEY. 



surface of the sac is lined with fine auditory hairs. The 

 sound waves coming through the air or water outside strike 

 against this sac, which lies in a hollow on the upper or 

 outer side of the antennae. The sound waves are taken up 

 by the contents of the sac and stimulate the fine hairs, 

 which in turn give this stimulus to the nerves which run 

 from them to the principal auditory nerve and thus to the 

 brain of the cray-fish. Among the insects other kinds of 

 auditory organs exist. The common locust or grasshopper 



