84 PRIMITIVE ANIMALS 



and that they are homologous to the Annelid ne- 

 phridia. 



In the true Vertebrates, these nephridia have 

 apparently disappeared without leaving a trace of 

 their existence, and they have been replaced by a 

 segmental series of coelomoducts, little tubes which 

 arise as outgrowths from the dorsal peritoneum, 

 and some of which become conglomerated to form 

 the adult kidney, while others take up an intimate 

 relation to the reproductive organs and function as 

 genital ducts. A similar interchange of function is 

 of frequent occurrence in the Annelids, in which 

 coelomoducts may be either genital or excretory in 

 nature. 



Our short conspectus of the structure of Am- 

 phioxus has shown us that whilst this animal is 

 closely related to the higher Vertebrates, yet in 

 other respects it strongly points to Annelidan 

 affinities, and even apart from Amphioxm we might 

 be tempted to associate the Annelids and Vertebrates 

 in a common origin from the profound similarity 

 which exists between them in the matter of their 

 metameric segmentation. 



We are led therefore on evidence of considerable 

 weight to adopt what is known as the Annelid theory 

 of Vertebrate descent. There is however a grave, 

 and to some thinkers an insuperable, objection to this 

 theory. It is that the relative position of the gut and 



