THE SPECIAL SENSES 225 



see, we shall have to base all our study on our own experi- 

 ence. We know of hearing and seeing only by what we 

 know of our own hearing and seeing ; but by examination 

 of the structure of the hearing and seeing organs of cer- 

 tain other animals, and by observation and experiments, 

 zoologists are convinced that some animals hear sounds 

 that we can not hear, and some see colors that we can 

 not see. 



While that phase of the study of the special senses 

 which concerns their structure may be quite successfully 

 undertaken, the physiological phase of the study of the 

 actual tasting and seeing and hearing of the lower animals 

 is a matter of much difficulty. The condition and char- 

 acter of the special senses vary notably among different 

 animals. There may even exist other special senses than 

 the ones we possess. Some zoologists believe that certain 

 marine animals possess a " density or pressure sense " 

 that is, a sense which enables them to tell approximately 

 how deep in the water they may be at any time. To 

 certain animals is ascribed a " temperature sense," and 

 some zoologists believe that what we call the homing in- 

 stinct of animals as shown by the homing pigeons and 

 honey-bees and other animals, depends on their possession 

 of a special sense which man does not possess. Recent 

 experiments, however, seem to show that the homing of 

 pigeons depends on their keen sight. In numerous animals 

 there exist, besides the organs of the five special senses 

 which we possess, organs whose structure compels us to be- 

 lieve them to be organs of special sense, but whose func- 

 tion is wholly unknown to us. Thus in the study of the 

 special senses we are made to see plainly that we can not 

 rely simply on our knowledge of our own body structure 

 for an understanding of the structure and functions of 

 other animals. 



118. Special senses of the simplest animals. In the Amoeba 

 (see Chapter I), that type of the simplest animals, with 

 16 



