MUSEUM OF COMPARATIVE ZOOLOGY. 319 



tion which resembles that by which Vertebrates acquire in their ontogeny 

 the segmental duct. 



From the facts thus far brought forward, I conclude, (1) that the 

 group of animals whicn presents nephridia most closely resembling those 

 of Vertebrates is unquestionably that of the CliBetopod Annelids ; and 

 (2) that the Vertebrate excretory system can be readily derived from 

 that of Annelids by a series of steps which are in accord with the evi- 

 dence afforded by the ontogeny of Vertebrates. 



In conclusion, I shall briefly allude to the opinions of previous writers 

 respecting the origin of the Vertebrate excretory organs. These opin- 

 ions fall, in the first place, into two classes, according to one of which the 

 excretory system is derived from Invertebrate ancestors ; according to 

 the other, it has arisen wholly within the Vertebrate phylum. 



The most recent exponent of the latter view is van "YVijhe ('89, pp. 

 50G et seq.). The arguments offered by this author in support of his 

 position are in part dependent upon his denial of the serial homology of 

 pronephros and mesonephros. Van W\jhe also employs two arguments 

 which are independent of his position in regard to this point: (!) ne- 

 phridia are absent in Amphioxus, and therefore the common ancestral 

 form cannot have possessed them ; (2) the renal organs do not appear 

 until after the so-called " Acrania stage," and therefore could not have 

 appeared phylogenetically until this stage had been passed. Granting 

 both premises, it seems to me that neither conclusion follows. With 

 reference to the absence of kidneys in Amphioxus, the possibility — or 

 should I not say probability 1 — of extensive degenerative modification is 

 entirely neglected. Moreover, it has now been rendered probable that 

 true nephridia do exist in Amphioxus, an observation which removes at 

 a blow the whole basis of the argument. I do not believe many embry- 

 ologists would unite with van Wijhe in holding that characters which 

 appear simultaneously in the ontogeny of a form must necessarily have 

 arisen contemporaneously in its ontogeny. For my part, I am unable to 

 diagnose with accuracy the "Acrania stage" of Amphibia; but Ostrou- 

 moff ('88, pp. 77, 78) appears to have had the necessary insight, and 

 denies emphatically that the pronephros in the case of Eeptiles ai'ises 

 after the " Acrania stage." 



Turning now to the hypotheses that have been advanced involving 

 the derivation of the Vertebrate excretory system from Invertebrate an- 

 cestors, Haeckel ('74, p. 37), Gegenbaur ('78, p. G28), and Fiirbringer 

 ('78 a , pp. 95 et seq.) endeavored to show that the Vertebrate nephridia 

 were derived from those of Plathelminthes. Semper and Balfour, on the 



