24 EARTHWORMS AND LEECHES [CH. I 



have more or less fallen in. Moreover, the excremen- 

 titious earth with which the burrows are lined is peculiarly 

 suited to the root fibres, being moist, loose and fertile. 

 Microscopic examination of the earth deposited by worms 

 shows it to resemble two-year-old leaf-mould such as 

 gardeners use for seed-pans and pricking-out young seed- 

 lings: most of the plant-cells are destroyed, shreds and 

 fragments alone remaining, discoloured and friable, mingled 

 with sand grains and brown organic particles. In chemical 

 composition too worm-castings are very similar to fertile 

 humus. 



The castings which are thrown up on the surface 

 materially improve the quality of the upper soil, and render 

 it the more fit for the germination of seeds, many of which 

 directly or indirectly get covered by the upturned earth. 

 It has been reckoned that there are upwards of 50,000 

 worms in an acre of soil of average quality : hence the 

 total effect of the work of this vast host must be very 

 considerable. Each worm ejects annually about 20 ozs. 

 of earth. The weights of earth thrown up in a single year 

 on two separate square yards observed by Darwin were 

 respectively G'75 Ibs. and S'SST Ibs., amounts which repre- 

 sent respectively 14'58 tons and 1812 tons per acre per 

 annum. 



In addition to this tilling action worms improve the 

 quality of the soil by the leaves and other organic debris 

 which they drag into their burrows, and thus bring within 

 reach of bacteria. These, as it is well-known, especially 

 abound in the upper soil, and effect the speedy decompo- 

 sition of dead animals and vegetable tissues. 



