PHYLUM ANNULATA 



469 



small ciliated funnels, are not known to communicate with the 

 exterior. 



In the Oligochseta the hephridia are usually simple, elongated 

 and coiled tubes, a pair or sometimes more than one pair in each 

 segment ; but, in some, these are replaced or supplemented in 

 certain of the segments, or, in all, by a branching system of tubes 

 with or without ciliated funnels. 

 Sometimes the ordinary neph- 

 ridia are not developed in the 

 segments lodging the reproduc- 

 tive organs, their place being 

 there taken by three pairs of 

 tubes of the nature of localised 

 ccelomoducts which become 

 modified to give rise to the 

 reproductive ducts ; but ordi- 

 nary nephridia may be present 

 in these segments as well. In 

 some Oligochaeta the nephridia 

 of the most anterior segments 

 open into the mouth or pharynx, 

 and have apparently taken on 

 the function of digestive glands 

 (peptonephridid), and aU the 

 nephridia of the posterior region 

 of the body in one species (Allo- 

 lobophora antipce), instead of 

 opening on the exterior, com- 

 municate with a pair of longi- 

 tudinal canals which posteriorly 

 open into a median vesicle 

 communicating with the rectum. 



The permanent nephridia of 

 the adult Chsetopod are pre- 

 ceded in the larva by pro- 

 visional or embryonic nephridia 

 of a temporary character. These 

 have been found to occur in the 

 head (prostomium) of many 

 larval Oligochseta and Poly- 

 chseta. They are ciliated in- 

 tracellular tubes, sometimes 



Fia. 383. Diagram to illustrate the various 

 combinations of closed and open nephridia 

 and coclomoducts in the Polychseta. 

 la, Hypothetical stage with closed nephridia 

 and separate coelomoducts ; b, condition in 

 which the ccelomoducts have become united 

 with the nephridia : this occurs in Phyllo- 

 docidce and Goniadidce ; c, condition in which 

 the cffilomoduct becomes reduced to a ciliated 

 organ (N ephthyidce) ; Ila, combination of 

 nephridia with nephrostomes and separate 

 ccelomoducts (Dasybranchus} ; b, condition in 

 which " segmental organs " are formed by the 

 union of nephridia with nephrostomes and 

 coelomoducts (the most usual condition) ; 

 c, condition in which there are nephridia with 

 nephrostomes, and the coelomoducts are 

 reduced to ciliated organs (Nereis, etc.). The 

 nephridia are outlined with a thick line : the 

 coelomoducts striated. (After Goodrich.) 



branched, which do not open 

 into the cavity of the prostomium. Sometimes solenocytes 

 occur at the inner ends of the branches or of the undivided tube. 

 Embryonic nephridia have also been shown to occur in the body 

 in certain forms. 



