ORDER TERRICOL^J; EARTH-WORMS. 309 



bristles, of which there are four pairs on each segment (Fig. 

 531, b). The hinder parts are then drawn forwards by a shorten- 

 ing of the body, a movement which the spines do not oppose ; 

 this swells out the anterior segments, and forcibly dilates the 

 passage into which the head had been already thrust. The 

 spines upon the hinder rings then take a firm hold upon the 

 walls of the hole into which they have been drawn ; .and this 

 part of the body being now made a fixed point, the head is again 

 forced forwards by the powerful contraction of another set of 

 muscles ; and by a repetition of this process, the animal easily 

 makes its way through substances, which it would at first have 

 seemed impossible for it to penetrate. The muscles, by which 

 this action is performed, are disposed in two series ; the arrange- 

 ment of which is very characteristic of the Articulata in general, 

 though it is less obvious in the higher classes, whose locomotion 

 is chiefly effected by the action of the extremities. The muscles 

 of one series are longitudinal in their direction, passing from 

 segment to segment, and serving by their contraction to draw 

 the segments together, in such a manner as to shorten the body, 

 at the same time increasing its diameter. The muscles of the 

 other series have a contrary direction, forming rings round 

 the body, and tending by their contraction to diminish its 

 diameter, and consequently to increase its length. 



844. The burrowing of Earth-worms is a process exceed- 

 ingly useful to the Gardener and Agriculturist; and these 

 animals are far more beneficial to Man in this way, than they 

 are injurious by devouring the vegetables set in the soil. They 

 give a kind of under-tillage to the land, performing the same 

 below ground that the spade does above for the garden, and the 

 plough for arable land ; and loosening the earth, so as to render 

 it permeable to air and water. It has been lately shown, that 

 they will even add to the depth of soil ; covering barren tracts 

 with a layer of productive mould. Thus, in fields which have been 

 overspread with lime, burnt marl, or cinders, these substances 

 are in time covered with finely-divided' soil, well adapted to the 

 support of vegetation. That this result, which is commonly 

 attributed by the farmers to the " working-down " of the 

 materials in question, is really due to the action of the Earth- 



