BRISTLE-FOOTED WORMS 



Class Ch^topoda. Annelids with Bristles 



245 



Segmented animals with setce developed in little skin-sacs, 

 either on a uniform body wall or on special locomotor pro' 

 trusions known as parapodia. The segments, indicated 

 externally by rings, are often marked internally by parti- 

 tions running across the body cavity, which is usually well 

 developed. The nervous system generally consists of a double 

 ventral chain of ganglia, connected with a pair of dorsal 

 cerebral ganglia by a ring round the beginning of the gut. 

 Two excretory tubes or nephridia are typically present in 

 each segment, and they or their modifications may also 

 function as reproductive ducts. The reproductive elements 

 are formed on the lining membrane of the body cavity. The 

 development is either direct or with a metamorphosis. 



Type of Oligoch^ta. The Earthworm (Lumbricus) 



Habits. — Earthworms eat their way through the ground, 

 and form definite burrows, which they often make more 

 comfortable by a Hning of leaves. The earth swallowed by 

 the burrowers is reduced to powder in the gut, and, robbed 

 of some of its decaying vegetable matter, is discharged on 

 the surface as the familiar " worm-castings." By the 

 burrowing the earth is loosened, and ways are opened for 

 plant-roots and rain-drops ; the internal bruising reduces 

 mineral matter to more useful form ; while, in covering 

 the surface with earth brought up from beneath, the earth- 

 worms have been ploughers before the plough. Darwin 

 calculated that there were on an average over 53,000 

 earthworms in an acre of garden ground, that 10 tons of 

 soil per acre pass annually through their bodies, and that 

 they cover the surface with earth at the rate of 3 in. in 

 fifteen years. He was therefore led to the conclusion that 

 earthworms have been the great soil-makers, or, more 

 precisely, that the formation of vegetable mould was mainly 

 to be placed to their credit. 



Though without eyes, earthworms are sensitive to light 

 and persistently avoid it, remaining underground during 

 the day, unless rain floods their burrows, and reserving 



