736 EDWIN S. GOODRICH. 



internal end.^ The nephridium of the first segment, the so- 

 called " head kidney/' in the majority of Polychaetes never 

 passes beyond that stage. The discovery of solenoeytes in 

 the " head kidney " of Polygordius is of some importance in 

 this connection (see p. 717). 



As for the second type of nephridium, it appears to have 

 been developed within the Polychfete group by the opening 

 of the true nephridium into the coelom. This is actually 

 what happens in the development of Nereis (Meyer, 22). 

 Such an opinion may, perhaps, commit us to the conclusion 

 that the nephridium has acquired an opening into the 

 ccelom independently in the Oligochtetes and the Polychsetes. 

 This would be an objection, no doubt, but, whatever may 

 be thought of this difficulty, it is clearly much less than the 

 many difficulties to be met by the opposite view. 



The third type of organ has been already dealt with (p. 

 728) and explained as the result of the fusion of the genital 

 funnel with the nephridium. 



The strictly provisional conclusion we have arrived, at, 

 therefore, is that, although the third type of excretory organ 

 may conceivably have been derived from the second, the first 

 type must represent a survival, possibly highly specialised, of 

 the protonephridial ancestral form without communication 

 with the ccelom. 



The Solenoeytes. — Lastly, let us discuss those peculiar 

 structures which I have termed solenoeytes. Typically they 

 consist of a cell-body, containing a deeply staining rounded 

 or oval nucleus, attached by a sort of neck to the extremity 

 of a thin tube which opens at its opposite end into the lumen 

 of the nephridial canal. The tube may be long, narrow, and 

 of almost even diameter, as in most Phyllodocids (figs. 3 

 and 9) ; or comparatively short, conical, and compressed with 

 fluted sides, as in Glycera (Part II, figs. 9 and 14). It 

 appears to be formed of a dense cuticular substance, and 

 may project far into the lumen of the nephridium (figs. 2 

 and 9). Working inside the tube and attached at its distal 



' lu Oligocbsetes and Hirudinea, as well as PolycLsetes. 



