STRUCTURE OF EXCRETORY ORGANS OF AMPHIOXUS. 499 



It results from these observations, as I have ah'eady pointed 

 out in a preliminary paper (7), that in their segmental 

 arrangement, in their function, and in their histo- 

 logical structure, the excretory organs of Am- 

 phioxLis and the nephridia of Phyllodoce are in all 

 essentials identical. 



Before committing ourselves to new theories, something 

 must be known of the development of these organs ; but, 

 considering how remarkably close is the agreement between 

 the two, it seems more than probable that they are homologous 

 structures. If two such excretory organs as the solenocyte- 

 bearing nephridia of Phyllodoce, and the solenocyte-bearing 

 kidneys of Amphioxus, could be shown to have been inde- 

 pendently involved, we should have to give up structural 

 resemblance as a guide to homology.^ But there seems to be 

 no danger of our being driven to abandon the problem as 

 yet, and all we need assume is, not that the vertebrates have 

 been evolved from the Polych^tes, but that the remote 

 common ancestor of these now highly differentiated phyla was 

 of more elaborate structure than most authors have been hither- 

 to inclined to suppose. We must assume that it possessed not 

 only paired coelomic (genital) sacs and coelomostomes (6 and 

 8), but also nephridia, whose blind internal end was provided 

 with solenocytes." We may conclude provisionally that 

 now, for the first time, true nephridia have been shown to 

 occur in the vertebrate phylum ; and further, we may hope 

 to trace in the vertebrates the same two series of organs — 

 the nephridium and the coelomostome — which I have elsewhere 



' The only case which seems to me at all comparable is that of the 

 iiematocysts in Coelenterates, Planarians, and Molluscs. 



'^ This is all the more easy to believe since I have found these cells at the 

 blind inner end of the nephridium of the larva of Phoronis (it will be remem- 

 bered that Masterman observed cells similar to Boveri's " fadenzellen" in 

 Actinotrocha [9], and they have been described by Wagener [12] ) ; and 

 " flame-cells " very like solenocytes have been described iu Nemertines by 

 Biirger (4), iu Molluscs by Meisenheimer (10), and in Rotifers by 

 Shepbard (11). Also the "flame-cells" of Platyhelminths and Entoproctous 

 Polyzoa are probably of the same nature (6). 



