PHYLUM ANNULATA 



460 



small ciliated funnels, are not known to communicate with the 

 exterior. 



In the Oligochseta the nephridia 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 Oligochseta the nephridia 

 of the most anterior segments 

 6*pen into the mouth or pharynx, 

 and have apparently taken on 

 the function of digestive glands 

 (peptonephridia), and afl. 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 



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. 



FIG. 383. Diagranito illustrate the various 

 combinations of closed and open nephridia 

 and coelomoducts in the Polychseta. 

 la, Hypothetical stage with closed nephridia 

 and separate ccelomoducts ; b, condition in 

 which the ccelomoducts have become united 

 with the nephridia : this occurs in Phyllo- 

 docidce and Goniadidce ; c, condition in which 

 the ccelomoduct becomes reduced to a ciliated 

 organ (N ephthyida;) ; II, combination of 

 nephridia with nephrostomes and separate 

 coelomoducts (Dasybranchws) ; 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.) 



