CHM TO POD A . 



197 



Class CH.ETOPODA. Worms with Bristles. 



Segmented animals with seta, developed in little skin-sacs, 

 either on a uniform body wall or on special locomotor pro- 

 trusions known as parapodia. The segments, indicated 

 externally by rings, are often marked internally by parti- 

 tions running across the body cavity, which is usually well 

 developed. The nervous system generally consists of a double 

 ventral chain of ganglia, connected with a pair of dorsal or 

 cerebral centres, by means of a ring round the beginning of the 

 gut. Tivo excretory tubes or nephridia are typically present 

 in each segment, and they or their modifications may also 

 function as reproductive ducts. The reproductive elements are 

 formed on the lining membrane of the body cavity, and the 

 development is either direct or with a metamorphosis. 



The two chief orders of this class may be contrasted as 

 follows : 



OLIGOCH.*:TA, e.g. Earthworm. 



Poi.YCH.iiTA, e.g. Nereis. 



With no parapodia, and with relatively 

 few seta;. 



Without any "jaw" apparatus in the 

 pharynx. 



No tentacles or cirri. Gills in a few 

 forms. 



With complex hermaphrodite reproduc- 

 tive organs, limited in number and 

 definitely localised. 



Very often with a clitellum. 



Development direct. 



Living in fresh water or in the soil. 



With parapodia and with very numerous 

 setae. 



The pharynx is often armed with 

 "jaws." 



With tentacles and cirri, and often with 

 gills. 



Sexes usually separate, and reproduc- 

 tive organs simple. 



Never with a clitellum. 



A metamorphosis in development. 



Marine. 



Type of OLIGOCH/ETA. The Earthworm (Lumbricus). 



Habits. Earthworms eat their way through the ground, 

 and form definite burrows, which they often make more 

 comfortable by a lining of leaves. The earth swallowed by 

 the burrowers is reduced to powder in the gut, and, robbed 

 of some of its decaying vegetable matter, is discharged on 

 the surface as the familiar "worm- castings." By the 

 burrowing the earth is loosened, and ways are opened for 

 plant-roots and rain-drops ; the internal bruising reduces 



