G54 RAYMOND PEARL. 



and wave the anterior end about in the water, first 

 away from and then towards the source of stimulation. 

 As the head comes nearer to the end of the tube, where the 

 solution is strongest, it is more strongly stimulated, and 

 gives a definite negative reaction. As it gets out into the 

 weaker zone again it is stimulated to a positive reaction 

 once more. If the tube is now removed the specimen will, 

 in some cases, after a short time turn sharply towards the 

 place where it was, and move in that direction. In other 

 cases the negative reaction finally predominates. It not in- 

 frequently happens that in the earlier part of this reaction the 

 anterior end only moves very slightly towards, or very 

 slightly away from the stimulus, so that the body seems, at 

 first sight, to be fixed in one position. The planarian, in this 

 strenuous reaction, probably comes as near to the hypothe- 

 cated behaviour of the famous "Buridan's ass" as anything 

 is ever likely to in actual practice. 



TFo P^^' cent. — One specimen gave clearly marked positive 

 reaction in every case. Others as in the preceding solutions 

 (tV P^^' cent, and J^ per cent.). 



•rr^ij per cent. — Well-marked positive reaction. Specimens 

 give complete typical food reaction. 



In one case, with a small worm, I was able to produce 

 crawling- in a backward direction by continuous stimulation 

 of the anterior end in the middle line of the body with 

 ^'j^ per cent. ZnSOj. 



Summary. — The i-esults from solutions of salts of two 

 heavy metals are in accord with those obtained with other 

 chemicals. 



Other Salts. 



Sodium Chloride, ^ per cent, and j\y per cent. — Nega- 

 tive reaction ; distinct, but not as strongly nuirked as the 

 negative reaction to strong acids. 



yV per cent. — Weak negative reactions and weak positive 

 reactions in about equal numbers. Mauy of the trials pi-oduce 

 no response whatever. 



