ANIMAL PHYSIOLOGY 73 



-are of such complexity of structure that many pages 

 would be required for their description. But among the 

 lower or less highly organized animals the ears, or audi- 

 tory organs, are much simpler. 



In most animals the auditory organs show the common 

 characteristic of being wholly composed of, or having, 



FIG. 31. The auditory organ of a locust (Melanoplus sp. ). The large 

 clear part in the center of the figure is the thin tympanum, with the 

 auditory vesicle (small, black, pear-shaped spot) and auditory ganglion 

 (at left of vesicle and connected with it by a nerve) on its inner sur- 

 face. A spiracle at the side of the tympanum allows air to pass into 

 a chamber behind the tympanum so that the air pressure is the same 

 on both outer and inner surfaces of the tympanum. (Greatly magni- 

 fied; photo-micrograph by Geo. O. Mitchell.) 



as an essential part, a small sac filled with liquid in which 

 one or more tiny spherical hard bodies called otoliths are 

 held. This auditory sac is formed of, or lined internally 

 by, auditory cells, specialized nerve-cells, which often bear 



