50 THE LEECH. 



behind the middle of its length, and is oval in trans- 

 verse section, the dorsal surface being more convex 

 than the ventral. 



2. The annuli, or rings, into which the body is divided by a 



series of transverse grooves, are about ninety-five in 

 number, and occur throughout the whole length of 

 the body except its extreme ends. They bear small 

 papillae, which are more obvious when the body is in 

 the contracted condition. 



3. The colour differs greatly in different individuals. The 



dorsal surface is darker than the ventral, and is 

 usually marked by three pale longitudinal stripes on 

 each side, the middle and outer of which are inter- 

 rupted by dark dots. These dots are markedly 

 larger, especially in the middle stripe, at every fifth 

 annulus, the annulus so marked being the hindmost 

 of a somite. The most anterior of the five annuli 

 making up a somite bears a transverse ring of small 

 white dots. Towards the two ends of the body the 

 somites are shorter and contain fewer annuli. 



4. The suckers. 



a. The anterior sucker is oval, with the longer axis 



longitudinal. It is placed on the ventral surface 

 of the anterior end, and is cupped in the centre 

 to form the buccal cavity. It may be regarded 

 as formed by the fusion of the prostomium, a part 

 in front of the mouth, with the first two annuli of 

 the first somite. 



The prostomial portion is very commonly 

 folded down over the mouth as a kind of lip. 



b. The posterior sucker is circular, and larger than 



the anterior one. It is composed of six segments 

 fused together, and is imperforate. 



5. External openings. 



a. The mouth is a funnel-like depression in the anterior 

 sucker, at the bottom of which are the three jaws. 



