POLYCHAETA 



243 



Nephridia may exist in the same segments as coelomoducts but 

 be entirely separate in position and function. They may also occur 

 grafted together. In the Alciopidae, for instance, nephridia and 

 coelomoducts occur in most segments united as the nephromixium, as 

 shown in Fig. 180 II. In Nereis the functional segmental organ is 

 an open nephridium, but a rudiment of the coelomoduct, which does 

 not open to the exterior, the so-called ciliary organ, occurs in each 

 segment. 



,nephr. 



'.o.d.-- 



Fig. 180. Segmental organs of Polychaeta. After Goodrich. I, Transverse 

 section (right half) of body segment showing combinations of nephridia and 

 coelomoducts. A, Hypothetical. B, Phyllodocidae and Alciopidae. C, Neph- 

 thyidae and Glyceridae. D, Capitellidae. E, Capitellidae. F, Nereidae. 

 co.d. coelomoduct; cor. ciliary organ; nep. "closed" nephridium; nep.o. 

 "open" nephridium; nmx. nephromixium. II, Segmental organ of Vanadis 

 (Alciopidae). nephr. coelomic funnel; sol. solenocytes. 



Then again there may be a great difference between the nephridia 

 in different parts of the same worm. In the serpulids, terebellids and 

 other families there are one to three pairs of long segmental organs 

 situated anteriorly. In most of the segments behind there are short 

 funnels in the body wall which are open nephridia but serve for the 

 escape of the eggs and sperm. There is thus a division of labour 

 between the segmental organs in tubicolous worms : the anterior are 

 specialized for excretion, the posterior are genital ducts. 



The closed nephridium appears to be the most primitive type of 



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