82 EARTHWOKMS. 



to one another, and lie in opposite directions, so that the 

 opening of the reoeptaculum seminalis of one worm is op- 

 posite the clitellus of the other. During copulation the 

 sperm flows back along a longitudinal groove to the recepta- 

 culum of the other worm. The ova produced as the result 

 of the fertilisation undergo unequal segmentation. 



Earthworms live most of their time underground, burrowing 

 to a great depth, often down to the subsoil : by so doing they 

 let in moisture and air, and loosen the subsoil to a considerable 

 extent, thereby doing much good. They draw down into their 

 burrows numerous leaves and other vegetation, which they 

 devour in large quantities : they feed upon these leaves dur- 

 ing the daytime, and draw them down at night. We may often 

 see them partly protruding from the opening of their burrows. 

 Large quantities of earth are also eaten by these useful annehds, 

 which they pass out of their tunnels on to the surface as the 

 " casts " or worm-casts we see so abundantly on our lawns, &c., at 

 certain times. By so doing these annelids are constantly bring- 

 ing fresh soil to the surface, sometimes from a considerable depth, 

 and thus do immense good. To the workings of earthworms the 

 soil called humus is partly due, by the repeated outpouring of 

 their casts and the intermixed leaves that they are constantly 

 drawing down, and which soon become decayed. Humus is a 

 dark rich soil which covers the surface of the land. According 

 to Darwin, every year as much as ten tons of soil are passed 

 through their bodies and brought to the surface by them per 

 acre. Worms jjrepare the soil for seedlings of all kinds by 

 exposing the mould to the air, and by sifting out the large 

 stones. Bonos and debris are buried under their castings, and, 

 decaying, mix with the dead leaves, &c., that they draw beneath 

 the soil, thus augmenting its fertility. 



