EARTHWORM 239 



backward to the posterior end of the body showing a gangHon in each 

 metamere, each ganghon being really a fused pair. Nerves from each 

 ganglion supply the structures in that metamere. 



276. Behavior. — The earthworm possesses no special sense organs, 

 but it responds to several stimuli. The response to a mechanical stimulus 

 is positive when the stimulus is not too strong and is continuous but 

 negative when it is of contrary character. This leads the worms to 

 remain at rest when in their burrows and to seek the greatest amount 

 of contact with surrounding objects when moving about on the surface. 

 They will move from their places in their burrows when the ground is 

 struck a hard blow and, if they are close to the surface, will come out of 

 them. It is this type of reaction which leads earthworms to leave their 

 burrows during a heavy rain, when the earth is pounded by the falling 

 raindrops, and to crawl upon the surface where they are soon beaten into 

 insensibility and even killed. This explains why it is that after such a 

 rain dead worms are frequently found on the surface of the soil, while 

 others are crawling about seeking again to enter their burrows. Darwin 

 showed that they did not react to sounds but would respond to sound 

 vibrations as mechanical stimuli. This result was secured when a flower 

 pot containing soil in which were some earthworms was placed upon a 

 piano, thus providing a means by which the sound vibrations could be 

 communicated to the pot and to the soil. 



Earthworms respond positively to chemicals which indicate the pres- 

 ence of food and negatively to harmful substances. This reaction occurs 

 not only when the substance comes in contact with the body but also 

 when a substance with a pungent odor is still a httle distance away. 

 The reaction to moisture is always positive. 



Earthworms are sensitive to light in varying degree in different parts 

 of the body, the anterior end of the body being most sensitive, the 

 posterior part next, and the middle portion least. Slight differences in 

 light intensity are detected, and the worms will seek a region of faint 

 illumination in preference to one where the hght is strong. The effect 

 of these responses to strong and weak light tends to keep the worms in 

 their burrows during the daytime but leads them to emerge at night. 



The earthworm exhibits various physiological states which are deter- 

 mined not only by the state of metabolism within the body but also by 

 previous stimulation. After being repeatedly and violently stimulated 

 the nervous system is put into such a condition that even slight stimula- 

 tion causes a response out of all proportion to the strength of the stimulus. 



Earthworms remain near the surface of the soil only when it is moist, 

 and as the soil dries they gradually retire farther into the ground, remain- 

 ing below the upper hmit of moisture. This occurs regularly in the latter 

 part of the summer when the ground becomes dry. They return to the 

 surface again in the spring when the ground is full of moisture. At 



