WORMS. 209 



important owing to the large numbers in which they 

 are present in the soil, and for another reason besides 

 the one just given. Darwin shows that in many parts 

 of England a weight of ten tons of earth per acre 

 passes through the bodies of earthworms, and is 

 brought to the surface by them, so that in a few 

 years the entire humus-containing surface layer of 

 earth has passed through their bodies. They there- 

 fore prepare the soil in an excellent manner for the 

 growth of plants, by continually exposing it to the 

 air. They cause stones to sink in the soil by throwing 

 out at the top earth which naturally consists only of 

 particles small enough to pass through their intestines. 

 They play the part of gardeners by thoroughly 

 mixing together the particles of soil, and bury under 

 their castings, in a shorter time than one would 

 imagine, objects found on the surface of the soil 

 (bones, oyster-shells, dead animals, leaves). Plant- 

 food is quickly formed again from these buried 

 matters. Earthworms, therefore, play a very im- 

 portant part in the economy of nature ; indeed, many 

 regions, now occupied by luxuriant pastures or fertile 

 cornfields, would be a waste, had it not been for 

 them. Where, however, in damp spots they injure 

 seedlings, they may be collected, either during the day 

 after a warm rain, or in the evening, at which times 

 they lie half out of their burrows. If a decoction of 

 walnut leaves is poured into these, they will crawl 

 out. Enemies: Mole, shrews, hedgehog, toads and 

 frogs, ground beetles, rove beetles, mole crickets, centi- 

 pedes, etc. 



CLASS: NEMATELMINTHES (EOUND WORMS). 



Body cylindrical, unsegmented; body wall tough, 

 booklets or spines may be present on it, but deeply 

 imbedded bristles are always absent. The gut may 



