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Show 310 CONCLUSION. CnAP. vn. ticJos which they can swallow are left in it. They mingle the whole intimately together, like a gardener who prepares fine soil for his choicest plants. In this state it is well fitted to retain 1noisture and to absorb all soluble subslances, as well as for the process of nitrification. The bones of dead anirnals, tho harder parts of insects, the shells of landnloHuscs, leaves, twigs, &c., are before long all buried beneath the accumulated castings of worms, and are thus brougl1t in a more or less decayed state within reach of the roots of plants. Worms likewise drag an infinite nurnber of dead leaves and other p art~ of plants into their burrows, partly for the sake of plugging them up and partly as food. The leaves which are dragged into the burrows as food, after being torn into the fin e~t shreds, partially digested, and saturated w itb the intestinal and urinary secretions, are commingled .with much earth. This earth forms the dark coloured, rich humus which almost everywhere covers the surface of the land with a fairly welJ.defined layer or mantl e. Von Hensen* placed two worms in a vessel * 'Zcitschrift flir wisscnschaft. Zoolog.' B. xxviii. 1877, p. 3GO. • CnAr. vn. CONCLUSION. 311 18 inches in diameter, which was filled with sand, on which fallen leaves were strewed; and these were soon dragged into their burrows to a depth of 3 inches. After about G weeks an almost uniform layer of sand, a centimeter (·4 inch) in thickness, was con· verted into humus by having passed through the alimentary canals of t.hese two worms. It is believed by some persons that wormburrows, which often penetrate the ground almost perpendicularly to a depth of 5 or ti feet, materially aid in its drainage ; notwithstanding that the viscid castings piled over the mouths of the burrows prevent or check the rain-water directly entering them. They allow the air to penetrate deeply into the ground. They also greatly facilitate the down ward passage of roots of moderate size ; and these will be nourished by the humus with which the burrows are lined. Many seeds owe their germination to having been covered by castings; and others buried to a considerable depth beneath accumulated castings lie dormant, until at some future time they are accidental1y uncovered and germinate. |