20 



Elementary Zoology. 



bristles, or seta. These setas are disposed in a perfectly regular 

 way upon each segment, and have — with a few exceptions to 



be noted immediately — an iden- 

 tical arrangement in successive 

 segments. 



In each segment of the body 

 there are eight of these setae 

 disposed in couples, the indivi- 

 dual setae of which are more or 

 less closely related ; the greater, 

 or less distance which separates 

 the two setae of a couple is fre- 

 quently a specific distinction. 

 On the first segment of the body 

 the sets are totally absent, as 

 also upon a projection of the first 

 segment — the prostomium, or 

 buccal lobe —which overhangs 

 the mouth. All the other seg- 

 ments have setae. The worm's 

 body is covered externally by a 

 delicate transparent cuticle. If a 

 worm be allowed to macerate 

 for a short time this cuticle be- 

 comes easily detachable, and 

 when a portion is stripped off 

 the setae occasionally come away 

 with it. The setae, when thus 

 isolated, are seen to be of a 

 yellow colour, and to be re- 

 gularly curved like an elongated 

 S, or like the mathematical 

 sign f, the blunter end being 

 that which is implanted in the 

 body wall ; the sharply curved 

 hook in which it ends at the other extremity protruding 

 freely on to the exterior. Upon the clitellum the setae 

 are of a different kind; they are longer as well as rather 



T.PJ:^ 



