588 RAYMOND PEARL. 



live a reaction which gets it foodj or helps to^ is of far 

 greater importance for the survival of the individual than a 

 reaction which takes it out of dauger; for, so far as observa- 

 tion can show, the dangers it encounters are relatively few. 

 It does not move over large areas of territory, and, so far 

 as is known, it does not furnish a considerable part of the 

 food supply of any other organism. Altogether its chief 

 struggle for existence would seem to consist in obtaining 

 subsistence for itself, and for this the positive reaction to 

 mechanical stimuli would appear to be an important aid. 

 As will be shown later, the food reaction proper consists 

 largely of this same response. 



We may now pass to a consideration of the — 

 2. Reactions to Stimuli applied to the Middle 

 Region of the Body. — I use the term "middle region of 

 the body " to distinguish that portion extending from the 

 posterior border of the head to about the middle of the 

 pharynx. The separation of the body behind the head into 

 a " middle '^ and a " posterior " region is based entirely on 

 physiological considerations, and is not defined morpho- 

 logically. 



a. Reactions to Strong Stimuli. — Strong mechanical 

 stimulation of the middle region of the body along the lateral 

 edges causes, in the first instance, a local contraction of the 

 body in the immediate region of the stimulus. This local 

 contraction is well marked; much more distinct than that in 

 the head region. If the stimulus is sufficiently strong, and 

 especially if the stimulating point is applied to the edge 

 from above rather than from the side, the previous gliding 

 movement will be changed to crawling. This will continue 

 for a brief interval, usually from two to four crawling con- 

 tractions being given; then the animal will relapse again 

 into the glide, provided the stimulus is not repeated. In 

 the case of a strong stimulus applied to the side of the 

 middle region of the body, especially if the stimulus is 

 several times repeated, we get the negative reaction — a 

 turning away from the side stimulated — just as in ihe similar 



