156 



A COURSE ON ZOOLOGY. 



The leech, on the contrary, has neither bristles nor 

 hairs, but at each end of its body is a sucker, by which 

 locomotion is accomplished. The mouth is situated at 

 the bottom of the anterior sucker; it has three little 

 triangular jaws, each armed with two rows of sharp 

 teeth, that can cut through the human skin and that 

 of other animals, leaving a Y-shaped incision. There 

 are several species of leeches ; all are carnivorous, and 

 several are used in medicine, the most valuable being 

 known as the medicinal leech. To supply the demand 

 for leeches, these annelides are now bred in large numbers 

 in artificial ponds, where they can find conditions favor- 

 able to their multiplication. 



FIG. 89. 



THE MEDICINAL LEECH (Hirudo medicinalis). 



The manner of nutrition of the leech requires in the 

 structure of its digestive apparatus profound modifica- 

 tions of the similar organs that we have thus far exam- 

 ined. The simple tube that exists in nearly all worms 

 is replaced by a series of eleven pockets or chambers, 

 spread out on the sides and communicating successively 



