468 



ZOOLOGY 



SECT 



is situated (most Polychseta) or of the segment in front (all or 

 most Oligochseta, some Polychseta). The nephridia thus in such 

 cases effect a communication between the coelome and the exterior, 

 and serve to carry oft waste-products which have passed into the 

 ccelomic fluid ; but in many instances the cells lining the tube are 

 active in separating out such waste-matters, and are loaded with 

 granules and concretions. 



In many Polychseta, however, there is no ciliated coelornic 

 aperture, the tube ending blindly internally, such a blindly ending 

 uephridium (Fig. 382) being frequently branched. On the inner 

 extremities in such cases, or on other parts of the tube, are situated 

 a number of peculiarly modified cells, the solenocytes, sometimes 

 separate, sometimes united together in groups. Each of these is 

 a rounded cell lying in the coelome, and connected with the 

 nephridium by a long, slender, tubular process : through the 



lumen of the process extends 

 a single, extremely long, vibra- 

 tile flagellum, which may be 

 prolonged for some distance 

 in the interior of the nephri- 

 dium itself. The resemblance 

 between these solenocytes and 

 the flame-cells of Platyhel- 

 minthes will at once be 

 recognised. 



In the Polychseta another 

 set of segmentally repeated 

 structures are frequently inti- 

 mately connected with the 

 nephridia. These are a series 

 of pairs of ciliated funnels, 

 the ccelomoducts, opening 

 widely into the coelome, and, 

 in a typical case, communicating with the exterior. In Nereis 

 they are represented by the dorsal ciliated organ, and are not 

 known to open externally. When provided with external aper- 

 tures, as is usually the case, the coelornoducts act as the 

 efferent ducts for the sexual elements. In many of the Poly- 

 chseta 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. 383). In some families of Pplychasta 

 (Serpula and allies) there is a single pair of large nephridia in the 

 anterior region of the body, with smaller pairs in the posterior 

 segments, the former alone appearing to have an excretory function, 

 while the latter act exclusively as genital ducts. In Sternaspis 

 only a single pair of nephridia are present,'which, though they have 



nephridial 

 canal 



Fid. 382. Inner branched end of nephridium of 

 Phyllodoce paretti, showing the nephridial 

 canal and the solenocytes. (After Goodrich.) 



