482 SENSATION IN THE LOWER ANIMALS 



stimulus applied upon a given area of covering affects a 

 sensory cell, or cells which are themselves the termination 

 of touch nerve fibers and control the area stimulated. 



Taste and smell. — The sense organs of taste and smell 

 are very difficult to distinguish in the lower forms. When 

 found they consist usually of pits or depressions in the 

 body covering, lined with sensory cells which are ciliated, 

 and are the terminations of taste or smell fibers. Often 

 the location of these pits is the sole indicator as to their 

 function. 



In the higher forms of life these sensory cells are still 

 seen in the hair cells of the taste buds or olfactory buds. 

 They are, however, distributed in connection with different 

 organs with a view to putting them in the path of odors 

 or tastes. Thus, the ciliated taste buds of our tongue, 

 while very similar in structure to the olfactory cells, are in 

 a position to come in contact with food rather than odors, 

 while the cells of the nose are in the path of inspired air 

 which brings them odors rather than taste stimuli. 



Sensitiveness of these taste or smell cells varies greatly 

 in different animals. Many animals, for example, can 

 detect the presence of an enemy or of food by smell alone, 

 when man would be utterly unaware of its presence. 



Hearing. — In the lower forms, the organs of hearing 

 consist of vesicles or cavities which may be open to the 

 outside air or water or may be inclosed in the body cover- 

 ing. These vesicles are lined with sensory 'ciliated cells, 

 and filled with a fluid in which float usually one or more 

 particles of limestone formation called otoliths. Each of 

 the sensory cells is the termination of an auditory nerve 

 fiber, and when a sound wave strikes the body it sets in 

 vibration the fluid and otoliths of these cavities. This 



