240 ELEMENTS OF COMPARATIVE ZOOLOGY. 



known castings. In this way they work over the soil, and 

 are of immense value to agriculture, as Darwin has shown in a 

 most interesting volume on these lowly forms. Our earth- 

 worms are moderate in size, but in Africa, South America, 

 and Australia giant earthworms, four to six feet in length 

 and an inch in diameter, occur. 



In other Chaetopods (POLYCH^T^;) the bristles are 

 numerous in each segment, and 

 are borne on fleshy outgrowths 

 (parapodia) from the sides of the 

 body. The head bears fleshy 

 feelers or tentacles, there are fre- 

 quently horny jaws in the mouth, 

 while eyes are commonly present 

 structures which are lacking in 

 the Oligochaetes. The Poly- 

 chaetes are largely marine, and Fl ?- 112. Anterior end of 



> * clam- worm (.ZVereis),showing 



occur in large numbers burrowing jays, eyes, tentacles, and 



bristle-bearing parapodia. 



in the mud of the shores or sea- 

 bottoms. Many of them are brightly colored, and marine 

 worms are among the most beautiful objects in nature. 

 They are largely carnivorous, and some of them are, to the 

 associated life, terrible animals of prey. 



ORDER II. HIRUDIKEI (Leeches). 



The leeches have the body-segments ringed, so that one 

 examining the outside would conclude that there were 

 more segments than are really present. There are no bris- 

 tles on the segments, but the hinder end always bears a 

 sucking disc, while usually there is a second sucker around 

 the mouth. The body-cavity is not distinct. There are 

 two great groups of leeches those with jaws around the 

 mouth, and those which lack jaws. 



