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