LEECHES. 65 



of the ground, where they soon become injurious, the softer parts of decaying vegetable 

 matter, and conveys beneath the soil the more woody fibres, where they moulder and 

 form the nutriment of living vegetation. Thus eminently serviceable to the agriculturist, 

 it likewise constitutes an indispensable article of food for innumerable creatures belonging 

 to every order of creation ; and perhaps is a solitary instance of an individual race sub- 

 jected to universal destruction. The very emmets seize it when disabled, and bear it 

 away as a prize; it constitutes throughout the year the food of many birds; fishes de- 

 vour it greedily ; the hedgehog eats it ; the mole pursues it unceasingly ; and, secured 

 as it appears to be by its residence in the earth from creatures inhabiting a different ele- 

 ment, many aquatic animals seem well acquainted with it, and prey on it as a natural 

 food. Frogs eat it, and it is even seized occasionally by the great water-beetle (Dylicus 

 marginalia] when used as a bait by the angler. Yet notwithstanding this prodigious 

 destruction, its increase is fully commensurate with the consumption, as if it was or- 

 dained to be the appointed food of all. 



The Naides (.\ "<'.>-)* The mud at the bottom of ponds and streams is frequently 

 perforated by Annelidans closely allied to the earth-worms. Their body is slender, and 

 the rings into which it is divided are few and but slightly marked. They commonly 

 live in their burrows, merely protruding their head, which is furnished with a long pro- 

 boscis, whereby they take their food, and for this purpose it is kept in constant motion. 

 These water-worms have a power of multiplication which is of a very surprising cha- 

 racter. One of the most common species in our brooks (A 7 ais proboscidea] consists, when 

 full grown, of about fourteen segments. After a time, however, new segments begin 

 to be formed a little in front of the tail ; these lengthen, and soon begin to separate 

 from the parent animal under the form of a new Mais, provided with proboscis, eye- 

 specks, and everything complete. Sometimes even before the newly-formed young has 

 quite broken off its connection with its parent, another generation is in course of pro- 

 duction near its own tail, and sometimes even this has begun to form a fourth before the 

 separation of the first is complete. 



The Leeches (Hirudo) are common in our ponds. 



The Medicinal Leech (Hintdo medicinalis), however, is not indigenous in this 

 country, but being easily obtainable, we shall select it as an example of the group 

 (Fig. 53). At each extremity of its body is a fleshy disk, which in progression acts as 

 a sucker; it can, moreover, swim with much elegance, but not with rapidity. Its mouth, 



FIG. 50. THROAT OF LEECH LAID OPE.V. Fiu. 60. TOOTH OF LEECH MAGNIFIED. 



situated in the middle of the front sucker, is furnished with three small semicircular 

 teeth, each provided with a saw-like edge. These teeth are placed in a tri-radiate manner, 

 so that when the action of the sucker has made the skin of its victim tense, their edges 

 are pressed against it with a saw-like movement until three cuts -are made, extending 



* Xais, a water nymph. 



