480 



ZOOLOGY 



SKCT. 



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 ccelomoducts, opening 

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



FIG. 377. Diagram to illustrate tlic various combinations of dosed and open nephridia and 

 coclomoducts in the Polych<eta. 



la, Hypothetical stage with closed nephridia and separate eelomoducts ; b, condition in 

 which the cuelomoducts have become united with the nephridia : this occurs in Pkyl/o- 

 docidte and Goniailidte ; c, condition in which the ctelonioduct becomes reduced to a ciliated 

 organ (Nephthyidit) ', II<t, combination of nephridia with nephrostomes and separate coelomo- 

 ducts (Dasybrancltus) ; b, condition in which " segmental organs " are formed by the union 

 of nephridia with nephrostomes and cnelomoducts (the most usual condition) ; i; 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 crelomoducts 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 

 Polychreta 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 Polychajta 



