HIRUDO. 



191 



The common leech (^Hirudo medicinalis) is an elon- 

 gated, slightly-flattened worm of freshwater habitat. It is 

 found commonly in ponds and is usually of a dark greenish 

 tint with yel- 



Fig. 117. — Ventral View of the 

 Leech. (Natural size. ) (Adnat.) 



Habits. 



External 

 Features. 



- Mouth. 



^Nephridiopore. 



Aperture 6 - 



-Annulus, 



rSegment. 



low Hnes and 

 spots. The ventral sur- 

 face is darker, sometimes 

 black. Like Arenicola, it 

 is piano-symmetric. The 

 mouth is at the anterior 



end in the 



centre of a 



sucker. It is 

 triangular and armed with 

 three chitinous teeth. At 

 the hind end is a large Aperture 9 

 posterior sucker and the 

 anus is situated dorsal to 

 it. 



As in Arenicola, we 

 can see a great number 

 of annuli or rings, of 

 which about five are 

 contained in each true 

 segment, but there is a 

 marked absence of gills 

 and appendages. Hirudo 

 breathes by the skin and 

 the suckers take the place 

 of appendages in loco- 

 motion. The segments 

 can be made out by the 

 presence on the first an- 

 nulus in each of paired 

 rows of sense-organs. In 

 this way there can be 

 found twenty- six seg- 

 ments, and development 



indicates that the posterior sucker is formed from seven 

 segments fused together. Hence the body of the leech 

 consists of thirty-three segments. 



^Suckei 



