142 LIFE, IN' ITS INTERMEDIATE FORMS. 
beset on two sides with numerous points 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when they are withdrawn into the interior of the foot ; 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 ( Lumbricus ), 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 (II 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. Geol. Soc., vol. ii. 
