216 



MANUAL OF ELEMENTARY ZOOLOGY 



the peristomium. 1 There are about 150 segments. At 



about one-third of the length of the body from its front 



,,, end, in segments 32-37 



inclusive, a glandular 

 thickening of the epider- 

 mis lies athwart the back 

 like a saddle and is often 

 mistaken for the scar of a 

 wound. This is known 

 as the clitellum. The 

 skin of the worm is 

 brownish above and paler 

 below ; it is covered with 

 a fine, tough, iridescent 

 cuticle secreted by the 

 underlying cells. In 

 every segment except the 

 first and the last there are 

 eight bristles or seta, 

 placed in two pairs on 

 each side, a lateral pair, 

 slightly above the middle 

 of the side, and a ventral 

 pair between the lateral 

 and the mid-ventral line. 

 The setas can be felt with 

 the fingers; they are 

 embedded in sacs of the 

 epidermis, by which they 

 are secreted, and to these 

 sacs are attached muscles, 

 by which they can be 

 moved. The setae are 

 organs of locomotion. 

 The worm extends its 

 body with the bristles 

 withdrawn. Then it 

 thrusts out those of the 

 forepart, so that they catch in the soil and hold firm while 



1 In the related Alloloboplwra the prostomium reaches only half-way 

 across the peristomium. 



Fig. 134.— A, The forepart of the 

 body of an earthworm (Lum- 

 bricus herculeus), from the 

 right side ; B, the first five 

 segments from below ; C, the 

 same from above ; D, worms in 

 coition. 



id., 



Band of mucus; elm. , clitellum; 

 l.set., lateral setas ; m., mouth ; per., 

 peristomium; pro., prostomium; s.2, 

 s.g, 5.15, s.26, 5.32, numbers of seg- 

 ments ; v.set., ventral setse ; o* , open- 

 ing of vas deferens ; ? , opening of 

 oviduct. 



