THE NEPHRIDIA OF THE POLYCH^TA. 717 



Fraipoutj but the hollow tubes have each a fiagellum working 

 inside towards the nephridial lumen, and a long and slender 

 " neck " of protoplasm reaches to the distal end of the tube, 

 where it is slightly enlarged, and gives off fine irregular 

 processes, obviously comparable to those in the nephridium 

 of Nephthys (11). With the instruments at my disposal in 

 Ceylon I was unfortunately unable to make out for certain 

 the exact relative position of the tubes, the protoplasmic 

 " necks," and the delicate web. Nor have I been able to 

 settle this difficult point with the help of sections. In fig. 48 

 these structures are drawn as I believe they really occur, but 

 this figure must be considered as to some extent provisional. 



It will be seen at once that if this interpretation be correct 

 the nephridium of the larva of Polygordius differs in one 

 respect very remarkably from the similar closed nephridia of 

 Nephthys, Grlycera, or Phyllodoce. Whilst in these there is 

 always one nucleus to each tube (forming one solenocyte), in 

 the larval kidney there are several tubes surrounding one 

 central nucleus.^ As to the presence of this one large nucleus 

 at the tip of each branch, my observations only confirm those 

 of Fraipont and Meyer. 



Notwithstanding this difference, the structures at the 

 inner extremities of the *^ head-kidney" of Polygor- 

 dius appear to be quite justly comparable to and 

 homologous with the solenocytes of Polychgetes. 

 And the recognition of this fact is of considerable importance. 

 Hatschek has shown (and Meyer confirms his observation) 

 that the second nephridium in the developing Polygordius 

 has the same internal end, with solenocyte-like structures. 

 That this second nephridium is strictly homologous with the 

 third nephridium of the succeeding segment there can be no 



* Professor Edward Meyer has very kindly written to me on this subject, 

 enclosing several figures of the " head-kidney " of Polygordius, some of which 

 are, 1 believe, not yet publisiied. Meyer confirms what is correct in the 

 descriptions of Hatschek and Fraipont, and has moreover discovered the 

 fiagellum which works down the tube. But this tube he believes to be merely 

 a canal in the process of the cell, a prolongation of the nephridial lumen. He 

 therefore does not distinguish between a "tube " and a " neck." 

 VOL. 43, PART 4. NEW SERIES. D D D 



