650 EAYMOND PEARL. 



In a few cases positive responses were produced^ but not 

 regu]ixv]y. 



otVtt psi' cent. — Positive reaction in all cases. The 

 whole food response was produced in case the end of the 

 tube was left in position. The worms '^gripped" it, glided 

 up on to it, and extruded the pharynx, in many cases running" 

 the latter up into the lumen of the tube. Anterior piece, 

 resulting from cutting animal in two transversel}^, acts like 

 whole worm (positive reaction), but less strongly. Decapitated 

 worm gave no response. In order to make sure that in this 

 case it was the extremely diluted acid which was producing 

 the result, numerous controls with distilled water and culture 

 water and fresh tap water were tried on the same speci- 

 mens, in alternation with trials with the acid. With tap 

 water and culture water the specimens were indifferent ; 

 but with the acid solution (-g-yVo P®^" cent.) mixed in either 

 tap water or distilled water they gave a well-marked positive 

 reaction. This showed clearly that the results were due to 

 the acid. 



Summary. — With the three mineral acids tested it was 

 found that to concentrations above a certain point the speci- 

 mens always gave the negative reaction, while to concentra- 

 tions below this point the positive reaction was given. The 

 absolute value of this " critical point " varies widely with 

 different individuals. The behaviour is essentially the 

 same as that in response to mechanical stimulation, viz. to 

 strong stimuli the negative reaction is given, to weak the 

 positive. 



Organic Acids. 



Oxalic, i per cent, and yL per cent. — Sharp negative 

 reaction. This solution affords a very strong stimulus 

 and quickly kills the specimen. The negative reaction 

 is very violent when once induced, but several speciuiens 

 were killed before they turned away. There was notice- 

 able a slight tendenc}^ to turn towards the stimulus the 

 instant it was perceived, and before this could be replaced by 



