THE NEPHRIDIA OF THE POLYCHJJTA. 739 



It may, therefore, be concluded that, so far as our present 

 knowledge allows us to judge, the closed nephridia and 

 solenocytes of the Polychaites are probably homo- 

 logous with the protonephridia and flame-cells of 

 the Nemer tines. 



The Importance of the Nephridium in Classifi- 

 cation. — Hitherto the classification of the Polychaeta has 

 been based chiefly on external resemblances, and the struc- 

 ture of the alimentary canal. That the nephridium and 

 genital funnel must also be taken into account, will, I think, 

 be owned by anyone who has read the description of these 

 organs given above. A superficial glance at the variations 

 occurring in their structure amongst the Polycligeta, might 

 perhaps lead one to suppose that the genital tunnels and 

 nephridia are too vai-iable to afford reliable material to the 

 systematist. The extraordinary plasticity of the nephridium 

 in the Lumbricidae, made known to us through the researches 

 of Perrier, Beddard, and Benham, might encourage this 

 view ; but it must be remembered that, even in the Lum- 

 bricidae, although the nephridia vary immensely in the 

 relative development of their parts, and in the number and 

 position of their openings, yet they throughout maintain a 

 definite Lumbricid character, a family resemblance. More- 

 over, in each of the other families of the Oligochaeta, the 

 nephridium assumes a more or less characteristic type of 

 structure, so that a worm could as surely be identified as 

 belonging to the Bnchytrseidae, for instance, by the exami- 

 nation of its nephridium, as by that of any other part of its 

 body (see Vejdovsky [26] and myself [9]). 



In the same way, amongst the Polych^ta, although im- 

 portant differences in the relative development of the parts 

 of the nephridium undoubtedly occur, yet the type of struc- 

 ture remains very constant within the families themselves. 

 For example, in the Lycoridea (Nereidee), the nephridium is 



the " head kidney" of Polygordius (p. 71G), siuce I have been unable to 

 ascertain for certain the exact detailed structure of these very aberrant 

 organs. 



