MORPHOLOGY OF ANIMALS 



among which may be mentioned mechanical, chemical, 

 thermal, and electrical, as well as light, gravity, etc. These 

 stimuli acting on the organism, start changes in the proto- 

 plasm (impulses) which are transmitted to portions of the 

 body distant from the point first stimulated and call forth 

 the coordinated activities of many different parts. In 

 higher animals there are special sense organs for receiving 

 certain of these stimuli and specialized protoplasmic 

 fibres (nerve fibres) for transmitting impulses, while nerve 

 centers (nerve cells) for coordinating activities appear 

 very far down in the animal scale. In the lowest animals, 

 however, there are neither nervous system nor sense 

 organs, and yet through the irritability of the general 

 protoplasm these functions are performed. 



In somic of the higher Protozoa there are specialized 

 parts of the protoplasm which serve for receiving and 

 transmitting stimuli, but in the lower forms of this 

 phylum these differentiations are lacking; and of course 

 there are no specialized sensory or nerve cells in any pro- 

 tozoan. The same is true of sponges, where none of the 

 cells are differentiated for receiving and transmitting 

 stimuli, i.e., there are no specialized sensory or nerve cells. 

 In all other phyla, however, certain cells of the body are 

 set apart for these particular functions, and the greater 

 the differentiation in these respects the more definite and 

 varied are the sensations, the more swiftly impulses are 

 transmitted to the motor system, and the more compli- 

 cated are the responses. 



The elements of the nervous system are nerve cells and 

 fibres, the latter being merely outgrowths of the former 



C 67 ] 



