24 THE ELEMENTARY NERVOUS SYSTEM 



developed as an effector gave occasion for the addition of 

 receptors or sensory elements after which the adjuster 

 or central nervous mechanism was differentiated. Hence 

 in dealing with the elementary nervous system it will not 

 be inappropriate to consider first such independent effec- 

 tors as are found in sponges after which the receptor- 

 effector system of the coelenterates may be dealt with, in 

 part as a system in itself and in part as the source of 

 the differentiated receptor-adjustor-effector systems of 

 the more complex invertebrates and the vertebrates. 

 Heretofore the approach to this subject has been chiefly 

 from the side of anatomy and histology; but important 

 as this line of research has been, the more recent ex- 

 perimental results have shown that the physiological side 

 of the question is quite as illuminating as the morpholog- 

 ical. In the following pages some of the more important 

 problems of the elementary nervous system will be dis- 

 cussed. While these problems will be taken up chiefly 

 from a functional standpoint, the structural aspect of the 

 questions concerned will not be omitted, for in discus- 

 sions of this kind the double viewpoint is more likely to 

 lead to sound conclusions than if only one aspect is kept 

 in sight (Parker, 1909, 1910 6, 1914 &). 



