NOTES ON EMBRYOLOGY AND CLASSIFICATION. 429 



This pair of ciliated funnels appears to be the same organ 

 in all cases.' Primarily it develops like the stomodeeum and 

 proctoda3um by an ingrowth of the ectoderm or deron. At 

 present no name is in use for this important pair of organs ; 

 they are spoken of as " segmental organs " in some groups, 

 as " primitive excretory organs " in others. Since very 

 usually these canals acquire an excretory function and give 

 rise to kidneys^ though they may also serve as genital ducts, 

 I propose to call them by the diminutive of the Greek word 

 for a kidney — namely, " nephridium." The nephridia in 

 Rotifers, and Turbellarians and Trematods, are the ciliated 

 canals, though in the flat-worms it is impossible to say where 

 in the canal system " nephridium " ends and " coelom " 

 begins. In Chaetopoda the nephridia are the segmental 

 organs, in Gephyrsea the pair of organs opening into the 

 cloaca, in Lamellibranchs they are the organs of Bojanus, 

 in Brachiopoda they are the oviducts (so-called hearts), in 

 Gasteropods they appear, at any rate, in the embryo, in many 

 cases (Urnieren). In tracheate Arthropods the Malpighian 

 filaments possibly are the nephridia, whilst the Vertebrate 

 kidney and genital ducts have recently been traced by Bal- 

 four and Semper to a series of nephridia. 



The terminology of the new doctrine as to the Vertebrate 

 genito-urinary canals appears to me to need some clearing 

 up, and I therefore submit the following : 



The metameric segmentation of the primitive Vertebrate 

 gives us a series of nephridia, derived from one single pair of 

 nephridia in the still earlier unsegmented Vertebrate. The 

 nephridia are not, however, in the metameric Vertebrate 

 separate from one another, each with its own external aper- 

 ture, as in Chaetopod worms, but all (on one side) are deve- 

 loped on a common stem or duct, so that they form one organ 

 on each side of the animal. This compound organ is a 

 kidney ; it may be called the " archinephron," its duct the 

 " archinephric duct.'^ By longitudinal fission parallel with 

 its axis, the archinephric duct splits into two — one, the 

 " pronephric duct,'^ in connection with the more anterior 

 nephridia which form the " pronephron ; " the other, " the 

 mesonephric duct," in connection Avith the posterior nephri- 

 dia, forming the " mesonephron." The pronephron (Kopf- 

 nieren) aborts, the pronephric duct becomes the oviduct ; it 

 is frequently called Miiller's duct. The mesonephron is the 

 Wolffian body, its duct the Wolffian duct. Its anterior 



^ Gegenbaur recognises two kinds of primitive excretory organs w))icli, 

 if really distinct, might be called 'anterior' and 'posterior nephridia' re- 

 spectively. 



