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). 
