PHYLUM ANNULATA 



479 



of ciliated cells, in the fluid contained in which there are one or 

 several calcareous otoliths. 



Ciliated grooves occur on the prostomium of many forms ; in 

 Aricia they are present on all the segments : they have a special 

 nerve-supply, but their function can only be conjectured. Tactile 

 cells of the epidermis, with or without a projecting tactile hair or 

 stiff cilium, are very common, especially on the prostomium in the 

 Oligocha^ta and on the tentacles and cirri in the Polycha'ta. 

 Groups of these are often aggregated together in papilhr or 

 gobht-lodies, with special nerve-supply and often with a ganglion 

 or a single nerve-cell at the base. 



The organs of excretion of the Cruetopoda are a series of 

 segmentally arranged tubes, the nephridia, of which a pair, as a 

 rule, occurs in each of the segments of the body with the exception 

 usually of a few at the anterior and a few at the posterior end. 

 In its simplest form the ncphridium is a curved tube, ectodermal 

 in origin, ciliated internally, opening on the exterior by a laterally 

 placed pore at the one extremity, and at the other ending 

 in a ciliated funnel or nepltrostomc, which opens into the coalome 

 either of the same segment as that on which the external aperture 

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

 or most OligochaBta, some Polychyeta). The nephridia thus in 

 such cases effect a communi- 

 cation 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 Polychasta, how- 

 ever, there is no ciliated 

 coalomic aperture, the tube 

 ending blindly internally, 

 such a blindly ending ne- 

 phridium (Fig. 376) 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 solcnocytcs, sometimes separate, some- 

 times 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, vibratile flagellum, which may 



~,solenccules 



nephridicd, 

 canal 



FK 



;. 370. Inner branched end of nephridium of 

 Phyllodoce paretti, showing the nephridial 

 canal and the solenocytes. (After Goodrich.) 



