CH/ETOPODA. 187 



leech-like type, this from a Polyclade Turbellarian, and this 

 from a Ctenophore. According to Sedgwick, the Annelids 

 are derived from an Actinozoon-like ancestor. But we 

 cannot here discuss these possibilities, nor the difficult 

 questions concerned with the meaning of segmentation or 

 metamerism. 



Class Ch/Etopoda. Worms with Bristles. 



Segfimited animals ivith setce developed in little skin sacs, 

 either on a U7iiforni body ivall or on special locomotor pro- 

 trusions known as parapodia. The segments, indicated 

 externally by rings, are often inarked internally by parti- 

 tions runnitig across the body cavity, which is usually well 

 developed. The nervous system ge?ierally 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 tvpically 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 cavitv, and the development 

 is either direct or ivith a metamorphosis. 



The two prominent divisions of this class may be con- 

 trasted as follows : — 



Ol[Goch,^ta, eg-., Earthworm. ' Polvch.^'ia, f.^., Xereis. 



With no parapodia, and with few seta;. 

 Other external appendages are also want- 

 ing, except that Branchiura has gills. 

 Hermaphrodite. 

 Development direct. 

 Living in fresh water or in the soil. 



With parapodia and with numerous scta^. 

 \Vith antenna;, gills, and cirri. 



Sexes usually separate. 



A metamorphosis in development. 



Marine. 



Type of Oligoch.eta. The Earthworm {Lunibricus). 



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 ways are opened for plant roots 

 and rain drops ; the internal bruising reduces mineral matter 

 to more useful form ; while, in burying the surface with earth 



