MOVEMENTS, KUC, OP FRESH-WATER PLANARIANS. 577 



sharpest point will slide off the body without giving any 

 effective stimulation. 



h. Description of Eeactions. — The reactions can best 

 be described by taking up in order the typical results 

 following stimulation of the different parts of the body. 



1. Stimulation of Head Region. — If a planarian 

 gliding along on the bottom of a dish be touched with a 

 needle on one side of the head^ it will, under normal circum- 

 stances, in the majority of cases, turn the head and anterior 

 one fourth of the body away from the side stimulated, and 

 continue gliding along in the new path determined by the 

 turning of the anterior end. This "turning away" reaction, 

 or, as we may call it for economy of words, negative reaction, 

 ^ will always be given if the stimulus is made sufficiently 

 strong. There is a certain intensity of stimulation below 

 which the negative reaction may or may not be produced, 

 depending on the physiological condition of the individual, 

 but above which it always occurs. If, again, a normally 

 gliding planarian be selected for stimulation, and this time 

 the stimulating point (preferably something finer and more 

 flexible than a needle) be touched very lightly to the 

 edges of the sides of the head or the auricles, we get, 

 provided the specimen is in the proper physiological con- 

 dition, a very graceful and striking reaction, quite different 

 from that obtained in the former case. This time the flat- 

 worm will stop for the briefest instant, turn the head and a little 

 of the anterior end of the body towards the side stimulated, 

 and at the same time raise the head from the bottom, until 

 finally the tip of the head points exactly towards the point 

 from which the stimulus came, and then glide forward in that 

 direction. This " turning towards " or positive reaction is 

 given only in response to very weak mechanical stimuli, and 

 then only when other conditions are favourable. It is a very 

 precise and characteristic reaction when it does appear. 



Having outlined the two main reactions following mechan- 

 ical stimulation in the head region, we may proceed to con- 

 sider each of them in more detail. 



