92 



THE AMERICAN NATURALIST 



[Vol. XL 



possesses as its function the collection and transfetence of food 

 to the pharynx. During the transformation of larval Ammocoetes 

 into the adult Petromyzon the organ is functionless. Beginning 

 with the adult Cyclostome, and from there on throughout the rest 

 of the vertebrate series, its remnant forms the thymus gland. 



As regards the excretory organs of amphioxus, the researches 

 of Boveri show that they are segmental in nature and that each 

 tubule opens upon the surface of the body, no collecting duct being 

 formed. Even on the theory of the annelidan origin of the ver- 

 tebrates this is a stage of development through which the verte- 

 brate ancestors must have passed, and instead of being an argu- 

 ment against the close genetic relationship of amphioxus to the 

 vertebrates above it, it is one of the best examples we have in all 

 zoology of the .persistence of an extremely primitive condition of 

 an organ, even after the general advancement of the body, in a 

 morphological sense, makes the presence of such segmental organs 

 appear out of place and not in harmony with the stage of develop- 

 ment of the organism as a whole. 



A similar instance of the persistence of primitive excretory 

 organs in the adult condition is furnished by Bdellostoma, the 

 only vertebrate which possesses a functional pronephros in the 

 adult condition, and when we compare the adult pronephros of the 

 Bdellostoma with the ontogenetic condition of the pronephros as 

 seen in mammals and in birds, we recognize at once that the dif- 

 ferences between these two stages are greater than the differences, 

 for example, which we find between the mesodermic segmentation 

 of amphioxus and other vertebrates or the segmentation of the 

 reproductive organs of the same two forms. 



So that this evidence, as well as all that I have previously brought 

 to notice, points to amphioxus as the iiearcst living form among the 



A glance at the itiformation coDtaincd in tlic table ^ivcn l)elow 



on the one hand, and with the hight>r vertebrates, on the other. 



Table of some of the primitive characters (p.) of BdeUostonia 

 which are embryonic (e.), for higher verteljrates. 



1. Xotochord. (p. e.) 



2. And its extension to the hypophysis, (p. e.) 



