350 NEREIS CHAP. 



is anterior, and the anus posterior ; there is usually an 

 extensive coelome ; and the nervous system and nephridia 

 are essentially similar to those of the earthworm. The class 

 Chatopoda ("bristle-footed" worms), in which the earth- 

 worms and their fresh-water and marine allies are included, 

 receives its name from the fact that all its members are 

 provided with cuticular setae, which in the order to which 

 the marine forms belong (Polych&ta) are usually long and of 

 varied forms, and are much more numerous than in the 

 earthworms and fresh- water worms, which constitute the 

 order Oligochceta. This order includes several families, 

 both Lumbricus and Allolobophora belonging to the family 

 Lumbricidcz. 



The oligochaetous earthworm, is, as we have seen, adapted 

 to a subterranean mode of life, and feeds on decaying 

 organic matter. For comparison, it will be instructive to 

 consider the external form and mode of life of a predacious 

 marine polychaetous worm, called Nereis, various species of 

 which are of common occurrence between tide-marks on 

 the sea-shore, under stones, and among sea-weed in all 

 parts of the world, sometimes swimming actively through 

 the water. The worm varies considerably in colour even in 

 the same species. 



In shape (Fig. 87) the body, which may be about 7 or 8 

 centimetres in length, is long and narrow, approximately 

 cylindrical, somewhat narrower towards the posterior end. 

 A very distinct head, bearing eyes and tentacles, is recog- 

 nisable at the anterior end ; the rest is divided by a series 

 of ring like narrow grooves into a corresponding series of 

 metameres, which are about eighty in number altogether ; 

 and each of these bears laterally a pair of movable muscular 

 processes called parapods, provided with bundles of setae. 



