CHAPTER XXVI. EXTERNAL CHARACTERS OF NEREIS. 



1. Nereis as Type of the Polychaeta. The Polychaeta 

 are so called because they possess large numbers of chaetae 

 or cuticular bristles, in contrast with the Oligochaeta, of 

 which the earthworm is a type, in which the chaetae are 

 few. The genus Nereis, of which there are several species, 

 is the most typical of the Polychaeta. It possesses, with 

 the exception of branchiae, almost all the structures and 

 characters which are found in any of the other forms, so 

 that they are most conveniently distinguished by the ab- 

 sence or modification of one or more of the complete set of 

 organs seen in Nereis. This applies more especially to the 

 external characters. Like the earthworm, Nereis is a 

 metamerically segmented animal, but it is more primitive 

 in that the segments are more similar to each other, it is 

 homomerous. There is no clitellum, and no restriction of 

 generative organs to certain anterior segments. The sexes 

 are separate and the generative organs are merely tracts of 

 the epithelium of the coelom, from which the generative 

 cells become detached to undergo development in the 

 coelomic cavity. There are no special generative ducts, 

 ova and spermatozoa escaping by dehiscence of the body- 

 wall. The only differentiation of the intestine is at the 

 anterior end, where there is an eversible pharynx, provided 

 with chitinous jaws and teeth, and a short oesophagus and 

 stomach. The Polychaeta are all marine. Nereis lives be- 

 tween tide marks and in shallow water, and either burrows 

 in soft ground or hides in crevices among stones and rocks. 



2. The Parapodia. The most characteristic external 

 features of Nereis are the lateral appendages known as 

 the parapodia. A pair of these is borne by every seg- 

 ment except the first and last. They are outgrowths of 



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