180 ANIMAL BIOLOGY 



shortening one portion of the body after another. On the ventral 

 region of the body are rows of bristles which aid in locomotion. The 

 bristles project backward when the worm is moving forward, and 

 so keep the animal from slipping backward when it lengthens itself. 

 The bristles also serve to hold the' animal in its burrow. 



Earthworms are of considerable value in the soil. They burrow 

 through the earth by swallowing the dirt which is mixed with vege- 

 table matter; both are then acted upon by digestive juices in the 

 aUmentary canal. The refuse of the food, which is not available 

 for use in the body, is ejected from the posterior end of the intestine. 

 The little piles of dirt that are sometimes so common on a, lawn are 

 the " castings " of earthworms. It has been found that soil worked 

 over by these animals is in better condition for the growth of plants. 

 Then, too, the deeper soil that has not been used by plants is brought 

 to the surface and mingled with the dirt recently used. Darwin * 

 estimated that in England earthworms annually bring to the top of 

 the ground eighteen tons of soU per acre. 



136. Suggestions for the study of the earthworm. — Laboratory 

 study. 



This study should be made upon living worms. The pupil 

 should first note and describe the general shape and segmentation 

 of the animal, the differences between the anterior and posterior 

 ends, the dorsal and ventral surfaces, and the characteristic appear- 

 ance of the girdle. An earthworm should be placed on a moist 

 surface such as soil or wet paper, and the locomotion of the animal 

 observed and described. A large specimen should be pulled, an- 

 terior end first, between the fingers, and the action of the bristles 

 noted and their situation and appearance studied with the help of 

 a magnifier. Touch the earthworm on various parts of the body, 

 and determine, if possible, which portions are the most sensitive. 

 Look on the dorsal and ventral surfaces for blood vessels, and watch 

 the pulsations of the blood in these vessels ; describe the location of 

 these blood vessels and state the direction in which blood flows in 

 each of them. 



'Darwin's "Vegetable Mold and Earthworms." 



