92 ELEMENTARY ZOOLOGY. 



impulse from (i) to (3) the central nervous system 

 lion cells) where the impulse is received and suitable 

 responsive impulses are started; (4) conductive tracts 

 (efferent nerves) which make the work of the central organ 

 of value by carrying an impulse to (5) some kind of cell 

 which acts, as a muscle, a. gland, a light-producing organ, 

 or an electric organ. Trace the connection of these 

 in Fig. 1 6. 



The nervous apparatus may be much more complex 

 than is indicated here, but these parts are essential. The 

 special development and the location of the parts are very 

 different in the groups of animals, depending on the 

 differentiation of the animal in structure and in habits 

 and powers. As we ascend the scale of animals the very 

 greatest improvement occurs in (i), (3), and (5). The 

 sense organs become more varied, more localized, more 

 highly specialized, and more perfect in adjustment; the 

 central nervous system becomes more and more concen- 

 trated and more closely co-ordinated as well as more 

 complex; and the responding organs become more per- 

 fectly, adjusted to the kinds of work the organism needs 

 to do. 



98. The Organs of Special Sense. In the lowest organ- 

 isms the sensitiveness to the various external stimuli 

 that can excite protoplasm interactivity is diffused over 

 the whole surface of the body. As we ascend the scale 

 the tendency is for certain parts of the surface to become 

 especially sensitive to certain kinds of stimuli and to 

 lose their sensitiveness for the other kinds. 



Some of the most primitive and diffuse kinds of sen- 

 sitiveness are to light, to temperature, to the chemical 

 condition of the medium, and to touch. Touch and 



