Spatially Determined Reactions 151 



forward if the stimulus affects the posterior half of the body 

 (210). In general, a reaction of type (2) rather than type (i) 

 will occur in proportion to the degree in which an organism's 

 movements are coordinated and it tends to act as a whole. 



3. One of the prettiest examples of the most highly co- 

 ordinated form of response to a single localized stimulus, 

 namely, movement of some other part of the body toward 

 the point affected, is to be found in the swinging over of the 

 jellyfish's manubrium toward the spot on the bell touched 

 by food. "In the typical feeding reaction," says Yerkes, 

 "the manubrium bends toward the food. If during such 

 a movement the piece of food be moved to the opposite 

 side of the bell, the manubrium, too, in a few seconds will 

 bend in the opposite direction, that is, again toward the 

 food " (451). The sea urchin responds to mechanical 

 stimulation by moving the spines toward the place stimu- 

 lated (410). In the higher animals this form of reaction has 

 largely superseded other methods of adapting behavior to 

 a stimulus acting at a definite point. Where grasping 

 appendages exist, the obvious device is to move them toward 

 the point of stimulation in order either to seize or to remove 

 the object. This involves not merely that the effects of the 

 stimulus shall diffuse so as to involve general locomotor 

 movements, but that the effect shall be exerted very defi- 

 nitely upon a particular set of muscles in a particular way. 

 The "scratch-reflex" of mammals, and the reaction whereby 

 a frog rubs its hind leg on the spot of skin affected by a drop 

 of acid, are further examples. 



What can we say regarding the conscious accompani- 

 ment of the reactions described under these three heads? 

 When a stimulus applied at point a brings about a reaction 

 different from that produced by precisely the same stimulus 

 acting on point b, are the accompanying sensations different, 



