628 KAYMOND PEARL 



fi'uin the food is evidently esseutially the same thiug' as the 

 positive reaction given to weak uiecliauical stimuli. It con- 

 sists in a turning of the anterior end of the body towards 

 the source of the stimulus. There is no reason for supposing 

 that its mechanism is in any way diiferent from that of tlie 

 same reaction to mechanical stimuli, and hence this need not 

 be further discussed here. A question of prime importance 

 with regard to this positive reaction in response to chemical 

 stimuli, Avhicli was not taken up before, is — how well localised, 

 with reference to the stimulus, is the reaction ? or, in other 

 words, how precisely does the anterior end point towards the 

 source of the stimulus, — in this case food ? Have we here a 

 clear-cut orienting response ? In answer to this problem it 

 may be said that when the worm is only a short distance 

 from the food the response is very precise. The anterior 

 end is brought by the first positive reaction so as to point 

 exactly towards the meat, and as the worm glides ahead it 

 never misses it. This is true where the specimen is near 

 enough (usually within three quarters of its own length), so 

 that the stimulus which reaches it is a fairly strong one. 

 In case the worm is stimulated near the edge of a large 

 diffusion area when the stimulus is very weak, the first 

 reaction may not suffice to direct the animal straight towards 

 the food. In this case the behaviour is usually like that 

 shown in Fig. 28, in which the line B, B, B, represents the 

 effective margin of the diffusion area of the piece of food A. 

 (By " effective nuirgin " is meant the line outside of which no 

 effect is produced by the food on passing specimens.) The 

 first reaction which the worm gives on reaching this diffu- 

 sion area (Fig. 28, 1 and 2) is a weak positive one. It then 

 proceeds on the new path into tliis area, but not directly 

 towards the food. After a short time, however (Fig. 28, 3), 

 it is again stimulated to a })ositive reaction" (4). This time 

 both the stimulus and the reaction are stronger than before, 

 and the worm is directed more nearly towards the centre of 

 diilusion, but still not exactly. When it gets oj^posite the 

 food again (5) another positive reaction (0) is given, and this 



