36 THE TROPISMS 



contact with a solid rather than in any response to gravity. 

 This is notably the case in the fresh-water Hydra. In 

 Planaria there is an effort to keep the ventral surface in 

 contact with some object whatever position in relation to 

 gravity this may involve. Planaria maculata shows a 

 modification of the thigmotactic response which Pearl 

 has called goniotaxis. When placed in a dish these animals 

 form groups in the angle between the bottom and sides. 

 The amount of surface in contact with the solid is not 

 appreciably greater than when the Planarian is on a flat 

 surface, but what is sought by the animal is a situation such 

 that the body becomes bent at an angle. 



Thigmotaxis is a very common trait among worms in 

 general. The effort to get into holes or crevices, or to work 

 in under rocks, by which these forms secure protection from 

 their various enemies, is to a large extent a manifestation of 

 this type of reaction. Maxwell showed that if specimens of 

 Nereis, which are usually found in burrows in the sand 

 near the seashore, are placed in a dish with a number of 

 glass tubes just large enough for them to enter, they will 

 crawl into the tubes and remain there even when exposed 

 to direct sunlight which is strong enough to kill them. 



Most amphipods and many isopods are strongly thigmo- 

 tactic and tend to collect in crevices between rocks or among 

 masses of seaweed where they secure contact over a consider- 

 able surface of the body. Among insects the instinct to 

 creep into crevices is a common trait. Earwigs if given an 

 opportunity to wedge themselves under a glass plate will 

 remain there, in spite of their negative phototaxis, even 

 when exposed to strong light. The moth Amphipyra, al- 

 though positive in its reaction to light, will cease its photo- 

 tactic activities if given an opportunity to crawl under a 

 glass plate, where it will remain quiet (Loeb). 



