EAKTHWORMS, LEECHES, AND SEAWOKKS 8.5 



seldom appear on the surface unless it rains. In winter 

 they hibernate below frost line. 



The economic importance of earthworms. — Mr. Darwin 

 has shown that earthworms exercise a profound influence 

 upon the surface soil. They are constantly depositing their 

 "castings" upon tlie surface of the ground near the mouths 

 of their burrows. This earth has been lirought from con- 

 siderable depths and carries whatever fertilizing constituents 

 there may be at these depths ])elow the I'each of the roots 

 of ordinary plants. MoreoA'er, the earth, in passing through 

 the alimentar}' canal of the worm has l)een \\'(jrked o\'er 

 and its fertility increased. Therefore, eartlnvorms may f)e 

 looked upon, in general, as improA'ers of the soil. Darwin 

 estimated that, if the castings of earthworms were spread 

 uniformly over the surface of England, they would add two 

 tenths of an inch every year to tlie thickness of the lich 

 surface soil. 



Leeches. — In many of our streams, lakes, swamps, and 

 marshy places there exist segmented, half-parasitic worms, 

 knoT\Ti as leeches, or "blood-suckers." Their Itodies are 

 somewhat flattened and in some species, at least, are capable 

 of great distention. The laodies of most leeches have a 

 sucking disk at both the anterior and posterior ends. By 

 means of these suckers they cling firmly to the fiodies of 

 other animals. Sometimes the mouth is furnished with 

 teeth and sometimes not. In some leeches the cajjacity 

 of the crop is greatly increased Iw a series of pouches jjlaced 

 along the sides of the crop and communicating with it. In 

 this way the crop is able to hold a supply of food sufficient 

 to last the leech several months. 



In the olden times it was the custom, among physi- 

 cians, to "bleed" patients. For this purpose leeches were 



