288 THE EARTHWORM AND OTHER ANNULATA 



find a safe retreat. When the upper levels of the soil are moist and 

 the worms can work at the surface night after night, they secure 

 most of their food in this manner. In times of drought they avoid 

 the surface and must depend upon such nutrient material as may be 

 extracted from the soil that passes through the digestive tract as a 

 result of their mining operations at the lower levels. Leaves, 

 bits of grass, and other vegetable material are laid hold of by 

 the mouth, which acts as a sucker, and dragged toward the burrow 

 or devoured little by little. A small portion of a leaf, for exam- 

 ple, is smeared with the saliva-like fluid secreted by the mouth, 

 and after being thus softened is " nibbled " away by the sucking 

 action of the lips. In this manner small bits of material are 

 conveyed into the digestive tract. While most of its food consists 

 of vegetation, the earthworm feeds also upon animal matter. 

 In nature the bodies of insects and other small animals that have 

 died at the surface attract earthworms, while in the laboratory 

 bits of fresh meat, and even salt pork and bacon are attacked 

 greedily. 



Under favorable conditions, when the worms are coming to the 

 surface nightly for weeks at a time, the burrow is relatively 

 permanent and its uppermost portion may be strengthened by a 

 lining of minute pebbles and other particles, much as the walls 

 of an old-fashioned well are lined with stones. In times of abun- 

 dant moisture or when the ground is suddenly flooded by a heavy 

 rain, the worms may leave their burrows and be found free in 

 great numbers on the surface, as is sometimes seen the morning 

 after a shower. This accounts for the popular idea that they 

 " come down in the rain," for they are observed on pavements 

 or roadways where they have strayed in their nocturnal wander- 

 ing and from which they have not found a retreat with the coming 

 of daylight. The animal's negative responses to light, save in 

 very low intensities as at twilight, and to a dry atmosphere are, 

 therefore, the two most important factors in controlling its activ- 

 ities at the surface. Because of these responses it is nocturnal 

 in its habits and is active at the surface of the ground only in 

 times of normal moisture. The abundance of the worms in the 

 evening after a heavy rain following a dry period may have 

 been observed by the reader. 



Internal Organization. — The general body plan of the earth- 

 worm is shown in Fig. 137 D. A tubular body wall encloses a 



