142 



LIFE, IN ITS INTERMEDIATE FORMS. 



beset on two sides with numerous poiuts directed back- 

 wards. As these bundles of spears are retractile, however, 

 the tender flesh of the animal would be liable to laceration 

 when they are withdrawn into the interior of the foot ; 

 but a beautiful provision is made to meet this emergency. 

 Each of the barbed javelins is furnished with two mem- 

 braneous blades, between which it is enclosed, in repose ; 

 these prevent the points from coming into contact with 

 its own flesh, while they readily open, and offer no impe- 

 diment to the extrusion of the weapon. 



In the common Earth-worm (Lumhricus), the rings are 

 very numerous, and each ring is furnished with eight 

 retractile bristles, by means of which it traverses its long 

 burrows. Mr Charles Darwin has satisfactorily proved 

 that earth-worms are most valuable agents in fertilising 

 lands, especially in undisturbed pastures, gradually cover- 

 ing the surface with their casts, and thus forming a layer 

 of finely pulverised earth of the richest character. A 

 field which had been limed was examined after about 

 eighty years, when the lime was found to be evenly 

 covered, to the depth of thirteen inches, with this animal 

 mould.'" 



The Leeches (Hirudo, &c.) are not provided with bristles 

 for locomotion ; but a compensation is given them in the 

 form of a sucking disk at each extremity. They move, 

 as is well known, by the alternate adhesion and detach- 

 ment of each sucker. The Medicinal Leech {H. medici- 

 nalis) performs its useful office, under the concealment of 

 one of these suckers ; and hence its mode of action is not 

 generally known. Its mouth is furnished with three 



* Proceed. G-eol. Soc, vol. ii. 



