384 Evolution and Adaptation 



as it again moves forward, it continues to bend its head 

 away from the light, until it is crawling directly away from 

 the source. When the light first strikes the worm, the two 

 sides will be differently illuminated. This causes a bending 

 of the head, as it stretches forward, toward the side of less 

 illumination, and the bending is due to a stronger contrac- 

 tion of some of the muscles on the less illuminated side ; 

 at least the reaction appears to be due to a simple response 

 of this kind. When the body has been so far turned that 

 the two sides are equally illuminated, the muscles of the two 

 sides will contract equally, and the movement will be straight 

 forward and away from the light. If the reaction is as simple 

 as this (which is in principle the explanation advanced by 

 Loeb), the result is a simple reflex act, and need not involve 

 any consciousness or intentional action on the part of the 

 worm to crawl away from the light. In fact, the same reac- 

 tion takes place when the brain is removed, not so quickly or 

 definitely, it is true, but this may be due to the removal of 

 the anterior segments of the worm, in which part the skin 

 appears to be more sensitive to light than elsewhere. 



Another factor that plays an important role in the habits 

 of the earthworm is the response to contact, — the so-called 

 stereotropism. If, in crawling over a fiat surface, the worm 

 comes in contact with a crevice, it will crawl along it, and 

 refuse to leave until the end is reached. The contact holds 

 the worm as strongly as though it were actually pulled into 

 the crevice. It can be forced to leave a crevice only by 

 strong sunlight, and then it does not do so at once. If the 

 worm crawls into a small glass tube, it is also held there 

 by its response to contact, and the smaller the tube, the more 

 difficult is it to make the worm leave by throwing strong sun- 

 light upon it. 



Loeb has found that when winged aphids, the sexual 

 forms, are collected in a tube, and the tube is kept in a 

 room, the aphids crawl toward the light. This happens in 



