468 



ZOOLOGY 



SECT. 



* -, solen ccyles 



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

 most Oligochaeta, some Polychaeta). The nephridia thus in such 

 cases effect a communication between the ccelome and the exterior, 

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

 coelomic 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 ccelomic 

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

 nephridium (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 celMying in the ccelome, 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 ccelome, 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 ccelomoducts act as the 

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

 chaeta 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 Polychseta 

 (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 



FIG. 382. Inner branched end of nepliridium of 

 Fhyllodoce paretti, showing the nephridial 

 canal and the solenocytes. (After Goodrich.) 



