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 therefore 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). Plantfood is quickly formed again from these buried matters. Earthworms, therefore, play a very important 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, centipedes, etc. CLASS: =NEMATELMINTHES= (ROUND WORMS). Body cylindrical, unsegmented; body-wall tough, hooklets or spines may be present on it, but deeply imbedded bristles are always absent. The gut may be absent (some parasites), but this is exceptional. There are no special organs for circulation and respiration. Sexes distinct in the large majority. I will deal with only one of the orders belonging here. ORDER: =Nematoda= (THREAD WORMS). [Illustration: FIG. 128.—Diagrammatic transverse section through the body of a Thread Worm, internal organs being omitted: _p_, cuticle and skin; _q_, muscle layer; _a_, lateral lines; _b_, dorsal and ventral lines. ]