480 



ZOOLOGY 



SECT. 



be prolonged for some distance in the interior of the nephridium 

 itself. The resemblance between those solenocytes and the 

 flame-cells of Platyhelminthes will at once be recognised. 



In the Polychaeta another set of segmentally repeated structures 

 are frequently intimately connected with the riephridia. These 

 are a series of pairs of ciliated funnels, the coelomoducts, opening 

 widely into the coelome, and, in a typical case, communicating with 



FIG. 377. Diagram to illustrate the various combinations of closed and open nephridia and 

 coelomoducts in the Polych^ta. 



I a, Hypothetical stage with closed nephridia and separate welomoducts ; 1>, condition in 

 which the ccelomoducts have become united with the nephridia : this occurs in Pk.yilo- 

 docidtv and Goni<nlid<.<: ; c, condition in which the ccelomoduct becomes reduced to a ciliated 

 organ (Ncfikthyidn) ; II, combination of nephridia with nephrostomes and separate ccelomo- 

 clucts (Dasybranclius) ; l>, condition in which " segmeiital organs " are formed by the union 

 of nephridia with nephrostomes and ctelomoducts (the most usual condition) ; r, 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.) 



the exterior. In Nereis they are represented by the dorsal ciliated 

 organ, and are not known to open externally. When provided 

 with external apertures, as is usually the case, the coelomoducts 

 act as the efferent ducts for the sexual elements. In many of the 

 Polycha3ta they do not remain independent, but coalesce partially 

 or completely with the nephridia, and the functions of excretory 

 organs and reproductive ducts become combined in the one set of 

 ' segmental organs ' (Fig. 377). In some families of Polychseta 



