HIEUDINEA OR LEECHES. 93 



see them partly protruding from the opening of their burrows. 

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

 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 

 bringing fresh soil to the surface, sometimes from a considerable 

 depth, and thus do immense good. To the workings of earth- 

 worms the soil called humus is partly due, by the repeated out- 

 pouring 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 prepare the soil for seedlings of all 

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

 large stones. Bones 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. 



Aster and Plant Koot Worms (Enchytrceidce). 



Some of these worms are plant parasites — the so - called 

 EnchytrcBidm — which live in the soil and attack the roots of 

 plants. They are small worms, usually white in colour, and 

 can be told by the setse being arranged in rayed groups, not 

 singly as in Lumbricus. 



Hirudinea (Leeches). 



The last group of worms to examine are the Leeches. 

 Leeches are provided with a ventral sucker for attachment to 

 their host. They are all hermaphrodites, and parasitic during 

 some time of their life, but are chiefly ecto-parasites. The 

 body of the leech is ringed, but the outside annulations do not 

 corresi^ond to the internal divisions. There may be two suckers. 



