350 NEREIS CHAP. 



integument is soft ; the mouth 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 Chcztopoda 

 (''bristle-footed" worms), in which the earthworms 

 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 (Polychceta) 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 Lumbricidce. 



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 predaceous marine polychaetous worm, called 

 Nereis, various species of which are of common occur- 

 rence 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, approxi- 

 mately cylindrical, somewhat narrower towards the 

 posterior end. A very distinct head, bearing eyes and 

 tentacles, is recognisable 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. 



