WORMS. 241 



The jawless leeches are aquatic, and occur in fresh water ; 

 more rarely in the sea. They live largely upon fishes, 

 feeding upon the mucus covering the body. The jawed 

 leeches have three jaws radiating from the mouth, and each 

 jaw has its edge finely toothed. With these jaws they are 

 able to cut the skin of vertebrates, upon the blood of which 



FIG. 113. A tube-inhabiting Polychaete (Amphitrite). At the upper end are 

 the tentacles, and just below to the left the gills. 



they feed. This blood-sucking habit led to the use of 

 leeches in medicine in those days when it was believed that 

 if a man were sick his cure could be effected by still fur- 

 ther weakening him. Most of the jawed leeches live in 

 fresh water, but in the warmer parts of the Old World land 

 leeches occur in the moist forests, and these form almost 

 intolerable pests. 



