ARTHROPODA 97 



The earthworm belongs to the subclass Oligochceta, 

 the members of which have but few bristles on each 

 segment. 



Nereis lives in the sea, under rocks and among sea- 

 weeds. Like the earthworm, it has a distinctly seg- 

 mented body. There is a well denned head, bearing 

 sense organs, as eyes and tentacles. The throat is 

 provided with two protrusible jaws, by means of which 

 the worm seizes its food, often living prey (Fig. 215). 

 Each segment bears a pair qf flattened, paddlelike par- 

 apodia, which enable the worm to swim rapidly. The 

 arrangement of the digestive, nervous, and circulatory 

 systems is much like that seen in the earthworm. 



Nereis is a member of the subclass Polychceta, which 

 is characterized by the presence of numerous bristles on 

 each segment. 



The leeches are externally segmented, usually flat- 

 tened, and have a sucking disk at each end of the body. 

 The mouth is in the anterior disk and is provided with 

 three semi-circular, saw-toothed jaws, by means of which 

 the leech makes the incision through which it sucks the 

 blood of its prey. The disks are also used for locomo- 

 tion. The digestive system is very capacious, and some 

 leeches can live even if not fed more often than once in 

 two or three months. Leeches are generally fresh- 

 water animals, though some kinds are found in the sea 

 and others live on land. 



Branch X. ARTHROPODA 



This is larger than all the other branches put together, 

 as it includes the animals with jointed legs, such as 

 crabs and insects. These differ widely from the mol- 

 luscan type in having numerous segments, and in show- 

 ing a repetition of similar parts ; and from the worms 

 DODGE'S GEN. ZOOL. 7 



