The development of the urincgenital organs of the lamprey. 71 



While subscribing to the view that the pronephridia of Verte- 

 brates are homologous with Annelid nephridia, I would, nevertheless 

 point out one striking difference, not specially noted hitherto, between 

 the renal organs of the two phyla. This is a peculiarity in the 

 ciliation of the nephrostomes, or funnels both in the pro- and meso- 

 nephros. In Vertebrates each cell in these portions of the tubules 

 bears but a single powerful flagellum, whereas in Annelids the free 

 surface of each cell is covered with a great number of delicate cilia. 

 The uniflagellate condition of the funnel cells in Chordata has been 

 described and figured for Araphioxus (Boveri), Wlyxine (Maas), Petro- 

 myson (in this paper), Acipenser (Jungersen), Ichihyophis (Semon), 

 Crocodilus and Chelone (Wiedersheim) ; I may add that I have also 

 seen it in Urodeles {Amblystoma, Biemyctylus and Spelerpes). The 

 single flagellum, when seen under a high magnification appears to 

 consist of a number of agglutinated cilia, and may have arisen from 

 a condition like that of the Annelid. Be this as it may, however, the 

 wide spread and constant occurrence of this peculiarity indicates an 

 antiquity which must reach back to the common ancestor of the 

 Vertebrate phylum. 



When we extend our survey of the renal organs to embrace not 

 only the Annelids but the Arthropods and Mollusks as well, we are 

 struck by an interesting developmental analogy with the Vertebrates. 

 We find that the renal organs of members of all these various groups 

 illustrate in a somewhat similar manner the principle of the sub- 

 stitution of organs. The embryo or larval Annelid starts with a 

 transitory "head-kidney" which recalls the permanent renal system of 

 Plathelminths. For this "head-kidney" the metameric nephridia are 

 substituted as development progresses. Similarly in the Mollusk 

 tubular "Urnieren" are developed in the cephalic region and these after- 

 wards give way to the true excretory organs of the trunk. In Insects 

 (Orthoptera, Odonata, Hymenoptera) the Malpighian vessels, few in 

 number in the embryo, are subsequently supplanted or, at any rate, 

 reinforced by a great number of postembryonic vessels (see Wheeler, 

 1893, p. 547). In Vertebrates, finally, the substitution of a meso- 

 nephros for the pronephros in nearly all adult Anamniote Craniota, 

 and the further substitution of a metanephros for the mesonephros in 

 all adult Amniota, is often cited as a splendid instance of the sub- 

 stitution of organs. Indirectly the analogous substitutions here 

 referred to tell against the views of Balfour, Field and others, who 



