THE REFLEX ARC 249 



It is in general true that in bilateral animals the 

 chief receptors and the chief adjuster or central organ 

 are situated in the head region, so that the functional 

 direction of the reflex arcs is predominantly posterior, 

 at least primarily. Attention has already been called 

 to the fact that in various flatworms and annelids 

 excitation is effective chiefly in the posterior direction. 

 Stimulation of, or injury to, the head may produce 

 effects throughout the whole length of the body, but 

 stimulation of levels posterior to the head has relatively 

 little effect anterior to the level of stimulation and much 

 greater effect at more posterior levels. These facts 

 indicate that the main conduction paths, or at least 

 those which have to do with the body wall, function 

 chiefly in the posterior direction, and that the head, so 

 far as its relations to the rest of the body are concerned, 

 is to a considerable extent autocratic. Unquestionably 

 numerous local arcs exist in these forms, and it is by no 

 means necessary to assume that the axial relations are 

 the same for all organs of the body. As regards the 

 alimentary tract of Planaria, for example, there is some 

 evidence that the pharyngeal region, not the anterior 

 end, is the dominant region, though whether nervous 

 differentiation exists in the entoderm is not known. In 

 most annelids the localization in early stages of a 

 secondary growing region at the posterior end of the 

 body determines a secondary gradient opposite in 

 direction to the primary gradient and involving more or 

 less of the posterior body region (Hyman, 1916; Child 

 191 7 d). In many of the aquatic oligochaetes the pos- 

 terior end is more or less dominant over the levels 

 of this secondary gradient. The dominance of this 



