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SEGMENTED WORMS OR ANNELIDA. 



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 or 

 cerebral centres, by means of 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, and the 

 development is either direct or with a metamorphosis. 



The two prominent divisions of this class may be con- 

 trasted as follows : — 



Type of Oligoch^eta. The Earthworm (Lumbricus). 



Habits. — Earthworms eat their way through the ground, 

 and form definite burrows, which they often make more 

 comfortable by a lining 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 earthworms 

 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 ins. in fifteen 



