HIRUDINEA 71 



superficial layer would be enriched by passing through their 

 bodies in a few years. Their burrows may extend vertically or 

 obliquely for several feet underground, their depth depending 

 upon the distance of the moist soil from the surface. " They are 

 connected by underground tunnels, so that the soil is thoroughly 

 exposed to the chemical action of the gases and acids of the air 

 and water. "^ Thus the action of the earthworm has both a 

 chemical and a mechanical effect upon the soil. 



Leeches were formerly used very frequently by doctors when 

 bleeding was more often practised. They are still sometimes 

 thus used. They are raised in France for commercial purposes. 

 Swamps are stocked with them and they are fed upon old and 

 worn out farm animals. 



Important Biologic Facts. — This branch is distinguished from 

 all preceding groups by its metameric segmentation. The 

 excretory system is characterized by the peculiar nephridia. 

 There is a well-developed circulatory system and a circulating 

 fluid containing hemoglobin. In leeches eyes are found, while 

 the " goblet-shaped organs " in leeches and earthworms are 

 thought to be the seat of smell or taste. True appendages ap- 

 pear in the Nereis. 



The trochosphere larvae show relationship between Chaetopoda 

 and the Turbellaria and the Nemertinia. 



Classification. — 



Class. Examples. 



I. Chsetop'oda. Earthworms, Sand-worms. 



II. Gephyr'ea. Sipunculus. 



III. Hirudin'ea. Leeches. 



'Jackson and Daugherty, "Agriculture Through the Laboratory and 

 School Garden." 



