XVIII INSTINCTS AND TROPISMS 351 



ments shown by the former." Of these frogs nine showed 

 an unmistakable phototactic response, turning by the shortest 

 course to face the Hght, where they remained " for a con- 

 siderable period, usually terminated by a jump toward the 

 light." This reaction occurs when the exposed portions of 

 the brain are covered by an opaque object or a shadow cast 

 over the head. The skin, therefore, is in all probability the 

 organ which is sensitive to stimulation by light. In the 

 urodeles Triton cristatus and Proteus a7iguinus it has been 

 shown that the skin is similarly sensitive to light. 



Thigmotaxis. — Many animals tend to remain in situa- 

 tions which afford contact stimuli over a considerable 

 surface of the body. Such forms are called positively 

 thigmotactic ; those which avoid contact are called nega- 

 tively thigmotactic. The tendency so common among 

 insects and worms to crawl under stones and lie under 

 crevices, is to a great extent the manifestation of a thigmo- 

 tactic response, although, in some cases, it may be due in 

 part to a negative phototaxis or photopathy. 



Frogs often show a propensity to crawl under stones or 

 to get between objects, where they remain quiet. The same 

 tendency seems to be somewhat more marked in toads. 

 It is apparently stronger when the temperature is lowered. 

 Miss Torelle in experimenting with frogs placed in a jar of 

 cold water found that " when a rock was lowered into the 

 jar in such away that a small space was formed between it 

 and the wall of the jar, the frog crawled into this space and 

 remained there. When a space was formed between the 

 bottom of the jar and the rock, it crawled into that. This 

 was tested several times, and was also observed when the 

 temperature of the water in the aquarium in which the frogs 

 were kept was lowered to 10° C. and below. When this 

 was done, all the frogs responded, either by flattening their 



