ANNELIDA. 715 



an inhabitant of ponds and lakes, and in such conditions has no opportunities 

 for tasting the warm blood for which it develops such a liking when oppor- 

 tunity offers. This is really a remarkable fact, that the animal should be 

 placed in a position naturally in which its most natural tendencies should 

 remain unsatisfied. 



The progression of the leech is performed by undulating movements 

 and the prehensile action of the suckers head and tail. The eyes are ten 

 in number, near the mouth and at the top of the head. The mouth is 

 furnished with numerous tri-radiate teeth, but in some leeches they are not 

 sharp, as in the " medicinal " variety. It is known that both sexes are repre- 

 sented in every leech, but they are not self-reproductive. 



The earth-worm, so familiar to all, has been lately raised into import- 

 ance. It lives in clay, and bores its way through the ground. It feeds 

 upon organic matter contained in the earth, and when it has assimilated the 

 nourishing particles, it ejects the remainder in small heaps of soft dirt, which 

 are visible after rain particularly. The worm k by its burrowing, turns up 

 the land, and vastly increases its fertility. 



The earth-worm in its outer structure resembles the leech, but, as any one 

 will at once perceive, the worm is not furnished with suckers by which it 



Fig. 841. Wheel animalcule (rotifer vulgaris). 



can assist itself to move. Instead of these rounded terminals the worm is 

 finely pointed, and thus capable of boring its way through the earth. Pro- 

 gression is accomplished by moving first the head portion and then the next, 

 so that a regular series of movements is necessary. The minute spines or 

 bristles of the worm prevent its body being retracted by muscular effort. 



The vital organs are rather forward of the centre of the body, and so 

 if a worm be cut behind them it will survive and reproduce a tail. But the 

 portion cut off will not be found alive, nor is it capable of forming a new 

 perfect worm as generally supposed, 



There are many other orders of worms which we can only indicate viz., 

 the Tubicola, which surround themselves with a hard case ; the Errantia, 

 or sea-worms, sand -worms, etc., like the lobworm used for bait, and the 

 naiads of our fresh- water ponds, all of which are suited to the aquatic life they 

 lead. Indeed, of all the annelides, the earth-worm is the only specimen 

 that is suited for living upon land. As regards the last mentioned, we may 

 add that worms do not prey upon dead bodies as is so generally imagined. 

 They are vegetable feeders, and do not burrow very deeply. 



The transparent condition of the Rotifers renders them easy of observa- 

 tion under the microscope, and we find a nervous system, intestines, and a 

 developed stomach. They are fresh-water inhabitants. 



