EXTERNAL CHARACTERS 37 



I. EXTERNAL CHARACTERS. 



1. The shape varies greatly with the degree of elongation 



or contraction. The body is widest a little way 

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

 verse section, the dorsal surface being more convex 

 than the ventral. 



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



series of transverse grooves, are about ninety-five in 

 number, and extend the whole length of the body 

 except its extreme ends. They bear small papillae 

 which are more obvious when the body is in the con- 

 tracted condition. 



3. The colour varies greatly in diiferent 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 sur- 

 face 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 prosto- 

 mium, 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 separated by a slight con- 

 striction from the body and is imperforate. 



