MOVEMENTS, ETC., OF FRESH-WATER PLANARIANS. 631 



diffusion. There was no doubt of tlie cliaracter of the 

 reaction ; the head was raised and the body turned in the 

 usual manner of the positive reaction^ which one can never 

 mistake after once having- become familiar with it. The 

 specimen kept on in the path determined by this last 

 reaction (Fig. 29, 4), and passed entirely out of the region of 

 the food. Evidently in this the worm was stimulated very 

 weakly by a chemical, and the stimulus was nearly as strong 

 on one side of the body as on the other, and when the reflex 

 was set off it was on the wrong side of the body. This is 

 not the usual result of weak stimulation, and has been 

 observed in only two cases, but it serves very well to show 

 the decrease oE the power of localisation when the stimulus 

 is very weak. 



When, as fi'equeutl}' happens, the worm approaches the 

 food exactly head-on, the reaction usually consists merely 

 of that portion of the reflex expressed in the raising of the 

 head, while the worm keeps on in its straight path till it reaches 

 the food. The head may be waved from side to side slig'htly, 

 but the general direction of motion is not chang'ed. The 

 action evidently corresponds to the positive reaction following 

 weak mechanical stimulation of the dorsal surface of the head 

 in the middle line, as described above. In some cases, how- 

 ever, I have observed very active and hungry specimens of 

 Dendrocoelum, sp., which were going straight towards the 

 food, give a complete positive reaction and turn to one side 

 and start off in a new direction away from the food. This, 

 however, of course brought the specimen at once into a 

 position where the stimulus was acting unilaterally, and it 

 again gave a positive reaction, this time heading it again 

 for the food, which it usually reached without further 

 reaction. But in some cases I have observed the specimen 

 give so strong a reaction as to be taken almost directly 

 away from the food by the subsequent movement, and, 

 passing out of the ai'ea of diffusion, fail to reach it at all. 

 Specimens behaving in this way were ''wild" in their 

 general reactions. The responses were very vigorous, but 



