670 RAYMOND PEARL. 



greatly diminished. This can best be shown with mechanical 

 stimulation. In strong solutions of NaCl (10 per cent.) the 

 animals make no attempt to right themselves if placed with 

 their dorsal surfaces down. Another peculiar effect of strong 

 solutions of NaCl is to cause the extrusion of the pharynx. 

 This organ is thrust out of the body and extended to a much 

 greater length than is usual. Immersion of the animal in 

 weak solutions that cause the positive reaction — as^ for 

 example^ 1 per cent, sugar — has no definite effect on the 

 movements, but when in these solutions the animals will give 

 the positive reaction to tap water when the latter is applied 

 by the capillary tube method. Under such circumstances 

 contact with water is a slight environmental change, and acts 

 as a weak stimulus. 



III. Tliigmotaxis and the Righting Reaction. 



a. Tliigmotaxis. — If a specimen of Planaria is turned 

 over and placed dorsal side down on the bottom, it will 

 immediately right itself. This is done by a very characteristic 

 reaction, and is one of the first things to attract the attention 

 of one studying the behaviour of the organism. Loeb ('94, pp. 

 251 — 252) held that the righting reaction in the poly clad 

 Thysauozoon was due to the negative and positive tliig- 

 motaxis {" stereotropism ") of the dorsal and ventral surfaces 

 respectively. The evidence offered for this view was that 

 when the thigmotactic relations of these two surfaces were 

 reversed, the animal reacted strongly, and tliat this result 

 could not be due to any effect of gravitation, since the animal 

 assumed all possible relations to gravity, and kept them for 

 considerable periods of time. It seemed to me desirable to 

 get, if possible, some further evidence on this subject, and to 

 work out the mechanism of the righting reaction. 



That the dorsal surface of the animal is negatively thigmo- 

 tactic is certain, and can be shown in other ways than by 

 laying the animal on its dorsal surface. For example, if a 

 piece of cover-glass be gently laid on the dorsal surface of 



