THE WORM AND LEECH TKIBE. 315 



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 marl 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 X 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* 



