94 ELEMENTARY ZOOLOGY. 



99. Practical Library Exercises on the Special Senses. Lack 

 of space forbids an extended treatment of the senses here, but the 

 student should refer to the larger texts for figures and discussion 

 of these most important and interesting powers and organs of ani- 

 mals. See Fig. 17 for some simple end organs of sense. 



Why are taste and smell spoken of as chemical senses? Where 

 are they located in earthworms? Where in fishes? What are the 

 essential differences between taste and smell? What other value 

 beside recognizing food would these senses have? Is there any way 

 to prove that the senses of touch and of temperature are not depen- 

 dent on the same end organs in our own case? Are otocysts hearing 

 organs or equilibrium organs? What evidences do you find in the 

 literature? Would the power to recognize the pull of gravity or 

 sounds be more important to the lower organisms? Why? What 

 are the lowest organisms known to hear? Would the power to 

 make definite noises be an evidence that a given species has the 

 power of hearing sounds? Trace out the various steps in the 

 development of the more complex kinds of eyes from the simpler 

 types? What is the simplest light-perceiving organ? What are 

 the chief differences* between the "insect" eye and the "vertebrate" 

 eye? 



100. Summary. 



1. The simplest animals perform all the necessary 

 functions by means of protoplasm. 



2. In the higher animals all the cells retain for them- 

 selves the power of using food, of growth, of oxidation, 

 and of eliminating the waste products from itself. Cer- 

 tain special functions as circulation, motion, sensation, 

 and the like may be turned over to certain special groups 

 of cells. 



3 . Associations of cells of similar kinds are called tissues ; 

 associations of tissues to perform a more or less definite 

 work are called organs. 



4. The principal functions of animals and the organs 

 or systems of organs doing the work may be classed as 

 follows : 



