ON THE NEPHRIDIA OF THE POLYCH/ETA. 445 



there is considerable variation in the structure of the nephridia, 

 perhaps owing to the specimens being of different ages or in 

 different stages of maturity.^ 



The Ciliated Organ. — The ciliatedorgan inGl. unicornis 

 and siplionostoma forms a considerable part of the nephridial 

 complex. It is of essentially the same structure in both these 

 species, and can be seen in a dissected specimen as a thick 

 band running from the nephridial sac, in front of the septum, 

 to the body-wall near the base of the parapodium (figs. 1 

 and 2). 



This band is hollowed out on its upper and anterior surface 

 by a groove which runs longitudinally along it, becoming 

 deeper at its inner end, where it reaches the sac and passes 

 into its interior (figs. 16 and 30). 



Having entered the mouth of the sac, which aperture it 

 entirely surrounds^ the ciliated organ is bent back on itself 

 so as to extend into the ''caecum/' a region of the nephridial 

 sac which will be described farther on. 



Outside the sac the edges of the grooved ciliated organ are 

 drawn out into two pointed flaps guarding the opening (fig. 3, 

 fl,). This structure forms a sort of one-sided funnel, and from 

 the free edge inwards extend slight ridges (fig. 3, r.), more 

 conspicuous in living specimens, which probably represent in 

 a very rudimentary condition the high ridges characteristic of 

 the ciliated organs in so many forms, such as Nereis and 

 Hesione (3 and 4). 



The whole of the grooved surface of the organ is provided 

 with a dense covering of cilia, by the action of which floating 

 bodies are driven into the sac. The ciliated organ is formed 

 on its inner surface of ordinary ciliated columnar epithelium, 

 consisting of narrow cells with oval nuclei^ which is continuous 

 at the edges with the flat coelomic epithelium, by means of 

 which it is attached to the septum, muscles, and body-wall 

 along its course (figs. 16, 17, 26, 27, and 28). 



It should be noticed that the band-like grooved outer lip of 



* I have found the typical fixed solenocytes in quite small and apparently 

 young specimens of Gl. unicornis. 



