THE WORM AND LEECH TRIBE. a 
the soil through which they burrow, extract from it the 
greater part of the decaying vegetable matter it may 
contain, and reject the rest in a finely-divided state. In 
this manner a field manured with mar! has been covered, 
in the course of eighty years, with a bed of earth aver- 
aging thirteen inches in thickness.” 
539. The order Suctoria includes the Leech and its al- 
lies. The Leech is shaped much like the Earthworm, 
but has a very different mouth, and a different apparatus 
of locomotion. It has a sucker at each end of its body, 
and walks quite fast by fixing the anterior sucker, and 
then moving the posterior one up to it, and throwing the 
whole body forward from this. Its mode of walking is 
much like that of the Measure-worms (§ 479), though 
its instruments for attachment are different. It can also 
swim very well by a waving motion of the whole body. 
Its mouth is in the middle of the cavity of the anterior 
sucker. In it are three semicircular saws, which make 
the bite of the Leech. They are so arranged that they 
work from a central point outward, and make a wound 
of this A shape. The wound being made, the blood is 
drawn out by the sucker. 
540. The sixth class of the Articulata, that of the Ento- 
zoa, includes worms that live in the bodies of various an- 
imals, man among the rest. I will notice of this class 
only those very singular animals which appear to us like 
long horse’s hairs, and are called Hairworms. We see 
them in stagnant water or in moist places; but they are 
really inhabitants of the bodies of various insects, and 
only resort to the water to lay their eggs. If taken from 
the water and left to dry, they become stiff, horny threads, 
and appear to have no life; but put them into water again, 
and they are soon restored to activity. 
541. The remaining class, that of the Rotifera, or 
Wheel Animalcules, contains animals of very minute size, 
some of them being less than the five hundredth part of 
an inch in length. Their structure, which is very won- 
