CHjETOPODA. 



165 



Class DiscoPHORA or Hirudinea. (Leeches.) 



To the Annelid series are appended two classes — 

 Chsetognatha or Arrow- Worms, e.g., Sagitta. 

 Rotatoria or Rotifers. 



Class ChjEtopoda. Bristly Worms. 



Segmented animals with setae 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 partitions 

 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. 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, and 

 the development is either direct or with a metamorphosis. 



The two prominent divisions of this class may be con- 

 trasted as follows : — • 



OLIGOCH.ETA, e.g.. Earthworm. 



With no parapodia, and with few setae. 

 Other external appendages are also want- 

 ing. 

 Hermaphrodite. 

 Development direct. 

 Living m fresh water or in the soil. 



P0LYCH.ETA, e.ff.f Nereis. 



With parapodia and with numerous seta. 

 With antenna:, gills, and cirri. 



Sexes usually separate. 



A metamorphosis in development. 



Marine. 



Type of OligochjETa. — The Earthworm {Lumbricus). 



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 bur- 

 rowers 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 w^ays are opened for plant-roots 



