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EVOLUTION OF ANIMALS 



Part V 



Fig. 28.15. Right, the common bloodsucker, American Medicinal leech {Mac- 

 robdella decora). Length, full grown, four inches or more. The general color is 

 green, mottled and lined with black and orange; the underside is rich orange. This 

 is one of the few leeches that regularly take human blood. It attaches itself by its 

 rear sucker and explores the skin with its anterior end, then attaches the oral 

 sucker and makes three fine painless cuts by a rotary motion of its jaws. Left, 

 common brook leeches, Glossiphonia complanata: one with eggs attached. They 

 live upon snails and are commonly called "snail leeches." A brook leech, two 

 inches long, may have 40 or more young leeches attached to its underside, stretch- 

 ing out their bodies from beneath the parent as they ride. 



whole surface and if they happen to pass over the fingerprint, their excitement 

 shows that they detect it. In their native ponds, bloodsucking leeches are very 

 responsive to movement of the water. They will gather from all directions even 

 when one moves slowly and in high boots. They are also sharply responsive 

 to light. Some have one or several pairs of eyes on the head as well as light 

 perceptive organs on the segments. 



Structure. There are 34 segments in the body of a leech but these are not 

 clear-cut externally for each one is furrowed by two to five circular wrinkles 

 or rings. Many structures of leeches are essentially similar to those of earth- 

 worms but their muscles are much stronger. 



The bloodsucking leech (Hirudo medicinalis) has three sawlike teeth that 

 make a Y-shaped cut in the flesh. Glands in the wall of the pharynx secrete 



