86 ANNELIDA. 



tliroughout the year the food of many birds ; fishes devour 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 inha- 

 biting a different element, many aquatic animals seem well ac- 

 quainted with it, and prey on it as a natural food. Frogs eat it, 

 and it is even seized occasionally by the great water-beetle (Dyticus 

 marginalis), when used as a bait by the angler. Yet notwith- 

 standing this prodigious destruction, its increase is fully commen- 

 surate with the consumption, as if it was ordained to be the 

 appointed food of all. 



The Naides [Nais*). 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 proboscis, whereby they take their food, and for this purpose 

 it is kept in constant motion. These water worms have a power of 

 multiplication wliich is of a very surprising character. One of the 

 most common species in our brooks (Nais jyrohoscidea) consists, when 

 full grown, of about fourteen se?:ments. 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 Nais provided with proboscis, eye-specks, and eveiything 

 complete. Sometimes even before the newly-formed young has 

 quite broken oft' its connection with its parent, another generation is 

 in course of production near its own tail, and sometimes even this 

 has begun to form a fourth before the separation of the first is 

 complete. 



Tlie Leeches (Hirudo) are common in our ponds. 



The Medicinal Leech {Hirudo 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 disc, 

 which in progression acts as 

 a sucker : it can, moreover, swim 

 with much elegance but not 

 with rapidity. Its mouth, situ- 

 ated 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 wdien 

 tlie action of the sucker has 

 made the skin of its victim 

 Fig. 59— throat of leech laid open, tense, their edges are pressed 



against it with a saw-like move- 

 ment, until three cuts are made extending to some depth, and the 

 blood thus liberated is largely sucked into the capacious stomach. 



* Nais, a water-nymph. 



