VI ANNELIDA 79 



though this may occur, at any rate in some cases when only 

 four or five of the head segments are lost, even the oesophageal 

 nerve ring and "brain" being then re-formed. 

 Action of '^^^ value of the eflfect of the activities of 



Earth- worms on the soil can hardly be over-estimated. 



worms on It is most important for plant life that the soil 



■ should be kept loose and open, so that it can be 



readily penetrated by the air which is essential for the healthy 



growth of the roots, and also for the activity of the bacteria 



in the soil which prepare the food-salts in it for the plant. 



Worms, by their burrows, which are constantly falling in 

 and having to be replaced by new ones, loosen the earth and 

 make it possible for air to enter even into hard soils. Their 

 burrows also bring about a good drainage of the soil, pre- 

 venting it from becoming too wet and heavy for vegetation. 

 Further, by their habit of swallowing soil at different depths 

 and then ejecting it in a finely-divided state at the surface, 

 they prepare it in an excellent way for the growth of young 

 seedlings and shallow-rooted plants. 



They add to the richness of the earth by dragging down 

 leaves, which are then more rapidly decomposed than they 

 would have been on the surface. 



The work done by worms in bringing up the earth from 

 the lower layers of soil and spreading it on the surface in 

 the form of castings, was studied in much detail by Charles 

 Darwin, who gives in his book ^ a full account of the experi- 

 ments he performed. It is well known that any layer of 

 stones or lime left on the surface of a field, in time becomes 

 covered with a layer of rich dark earth, and Darwin examined 

 special cases of this with the following results : — 



(1) In a field of good pasture land, after nearly 15 years, 

 it was found that a layer of quicklime that had been spread 

 over the surface was now nearly 4 inches below it. Mould 

 of an average thickness of "22 of an inch had been brought 

 up annually by the worms. 



(2) A piece of swampy waste land was drained and 

 ploughed, covered with a layer of burnt marl and cinders and 

 sown with grass. After 21 years this layer was still dis- 

 tinguishable af a depth of 4 to 5 inches below the surface, the 

 average annual increase of the surface layer being "1 9 of an inch. 



^ Vegetable Mp-nlfl f^vd Earthworms, chap. iii. 



