PHYLUM ANNELIDA. 



Class CH^ETOPODA. Annelids with Bristles 



Segmented animals with set<z 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 

 cerebral ganglia by a ring round the beginning of the gut. 

 Two 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. The 

 . development is either direct or with a metamorphosis. 



The two chief orders of this class may be contrasted : 



OLIGOCH^ETA, e.g. Earthworm. 



A, e.g. Nereis. 



With no parapodia, and with relatively 

 few setae. 



Without any "jaw" apparatus in the 

 pharynx. 



Head not highly developed. No tent- 

 acles or cirri. Gills in a few forms. 



With complex hermaphrodite reproduc- 

 tive organs, limited in number and 

 definitely localised. 



Development direct. 



Living in fresh water or in the soil. 



With parapodia and with very numerous 

 setae. 



The pharynx is often armed with 

 "jaws." 



The head is much more developed, and 

 bears tentacles and cirri. Gills are 

 often present. 



Sexes usually separate, and reproduc- 

 tive organs simple. 



A metamorphosis in development. 

 Marine, with two or three exceptions. 



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 

 mineral matter to more useful form ; while, in covering 

 the surface with earth brought up from beneath, the earth- 



