POLYCHAETA 



277 



191 I, E the funnel of the coelomoduct has completely fused round 

 the opening of the nephridium. 



In Nereis, Nephthys and Glycera 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. In the majority of polychaets, however, the coelomo- 

 duct has disappeared altogether. 



co'.d. 



nephr. nepi 



nu. 



^~"^} — nep. 



Fig. 192. 



Fig. 192. Segmental organ of G/om/)/zoma (Hirudinea). After Oka. Showing 

 mesodermal part with ciliated nephrostome and a single cell of the ectodermal 

 part, with intracellular duct. nu. nucleus. 



Fig. 193. Development of Megascolides australis (Oligochaeta). At the 

 posterior end the nephridia are single ; traced anteriorly they break up into a 

 number of loops each of which becomes a separate micronephridium (nep.'). 

 al. gut; sep. septa. Other letters as in Fig. 191 for both figures. 



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. 



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