MOVEMENTS, ETC., OF FRESH-WATER PLANABIANS. 581 



whole organism are deterinined by definite stereotyped 

 reflexes, yet in such exceptional cases as the one just 

 described the organism as a whole takes control, and does 

 something quite different from what the normal reflex fitted 

 to the case wonld accomplish. 



Very sti-ong mechanical stimulation of the anterior end, 

 such as to wound the animal, causes a very much more 

 vigorous reaction than the ordinary negative one, and of a 

 slightly different form. The animal contracts strongly longi- 

 tudinally, and, as a result of the heavier musculature on the 

 ventral surface, curls the head in under the body. Then the 

 anterior end is turned to one side through a larger angle 

 than is usually the case, and the worm straightens out in this 

 new direction. The point of importance to be noted in this 

 reaction to maximal stimuli is the curling under of the head. 

 The turn away from the side stimulated frequently is so great 

 as to turn the animal squarely about, so that it heads in the 

 direction opposite to that before stimulation. Besides this 

 effect of maximal stimuli jnst desci-ibed, they may also pro- 

 duce a change in the movement from gliding to crawling. 

 The crawling does not usually follow stimulation of the 

 head end of the body, but it is possible in some cases to pro- 

 duce it by very strong stimulation here. I have also been 

 able in a normal animal to induce crawling backward by 

 very strong and continued stimulation of the anterior end of 

 the body. This backward crawling, Avhen it occurs, is of the 

 same character as the same movement in a forward direction, 

 except that all the factors are reversed. It has been 

 described above (cf. p. 551). It is much more easily pro- 

 duced after certain operative procedures, and in connection 

 with them further details regarding it will be brought out. 



The negative reaction, i.e. that to strong stimuli, is given 

 more frequently than any other in the course of the activity 

 of the individual, and apparently does not depend on the 

 presence of any special physiological condition. It is given 

 in response to stimuli covering a wide range of intensity. 

 The lower liminal value of the stimulus producing it (there 



