188 



SYSTEMATIC ZOOLOGY. 



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 



FIG. 31. Diagram of two Polyzoan individuals, one expanded, the other re- 

 tracted into its cell (after Delage et Herouard). cr, anus; m, mouth sur- 

 rounded by the tentacles; o, operculum which closes the cell; r, retractor 

 muscle. 



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

 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 



