264 GENERAL BIOLOGY 



upturned earth. It has been reckoned that there are upwards of 50,000 

 worms in an acre of soil of average quality : hence the total effect of 

 the work of this vast host must be very considerable. Each worm ejects 

 annually about 20 ozs. of earth. The weights of earth thrown up in a 

 single year on two separate square yards observed by Darwin were 

 respectively 6.75 and 8.387 Ibs., amounts which represent respectively 

 14.58 tons and 18.12 tons per acre per annum. 



"In addition to this tilling action worms improve the quality of the 

 soil by the leaves and other organic debris which they drag into their 

 burrows, and thus bring within reach of bacteria. These, as it is well 

 known, especially abound in the upper soil, and effect the speedy decom- 

 position of dead animals and vegetable tissues. 



"Archaeologists are indebted to worms for the preservation of many 

 ancient objects, such as coins, implements, ornaments, and even the 

 floors and remains of ancient buildings that have become buried by the 

 soil thrown up as worm-castings. The process of disappearance is of 

 course hastened by the excavations effected by the worms below the 

 surface, for the collapse of the burrows slowly but surely allows objects 

 on the surface to sink downwards. 



"In the disinteg'ration of rocks, and the denudation of the land, 

 worms play an important part. The penetration of the burrows, and 

 the lining with castings, carries down the humus-acids to a considerable 

 depth and exposes the underlying rocks to their solvent action. Within 

 the body of the worm itself small stones and grains of sand are reduced 

 to yet finer dimensions and rendered the more easy to transport by wind 

 and water. On sloping surfaces the upturned castings, at first semi- 

 fluid, flow down, and when dry roll down the incline, or are washed by 

 the rain into the valleys and ultimately carried out to sea, while on 

 level ground the dried castings are blown away to lower spots by the 

 wind. The more or less parallel ridges that are frequently found on 

 ' the sloping sides of grass-clad hills are in part, at any rate, formed by 

 the material derived from worm-castings, which has temporarily lodged 

 against tufts of grass, etc., and in turn furnishes a richer and deeper soil 

 for stronger growth which arrests yet more and so increases the ledge. 

 All land surfaces, whether level or sloping, provided they are occupied 

 by worms, are reduced in altitude by their action. In no small degree, 

 then, may earthworms be held responsible for our valleys and hills and 

 all the softer features of our scenery." 



EXTERNAL APPEARANCE 



There are rings or segments (Fig. 165) extending along the entire 

 length of the animal's body formed by constrictions or annuli. The seg- 

 ments themselves are known as somites or metameres. It is from these 

 ring-like (L. Annulus-ring) constrictions and segments that the animals 

 belonging to this group are named Annelids or Annulata. Worms are 



