EARTHWOEMS, LEECHES, AND SEA WORKS 85 



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 the surface of the ground near the mouths 

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

 siderable depths and carries whatever fertilizing constituents 

 there may be at these depths below the reach of the roots 

 of ordinary plants. Moreover, the earth, in passing through 

 the alimentary canal of the worm has been worked over 

 and its fertility increased. Therefore, earthworms may be 

 looked upon, in general, as improvers 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 the thickness of the rich 

 surface soil. 



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

 marshy places there exist segmented, half-parasitic worms, 

 known as leeches, or "blood-suckers." Their bodies are 

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

 of great distention. The bodies of most leeches have a 

 sucking disk at both the anterior and posterior ends. By 

 means of these suckers they cling firmly to the bodies of 

 other animals. Sometimes the mouth is furnished with 

 teeth and sometimes not. In some leeches the capacity 

 of the crop is greatly increased by a series of pouches placed 

 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 



