82 ANATOMY OF AMPHIOXUS. 



and that of the Vertebrates, on the other, took their 



origin. 



In view of the apparent absence of nephridial tubules 

 in Balanoglossus and the fact that in the Ascidians the 

 renal organs are special structures peculiar to this group, 

 it is extremely difficult to associate the Vertebrate type 

 of excretory system with that of any Invertebrate. 



Since the Annelid-theory precludes the possibility of 

 Amphioxus being regarded as an ancestral form, and yet 

 if, nevertheless, it is, as we believe, primitive and not 

 essentially degenerate, the discovery of the excretory 

 tubules in Amphioxus happily releases us not only from 

 necessity, but also from the possibility of referring the 

 Vertebrate excretory system back to that of the Annelids. 



Nervous System. 



The central nervous system of Amphioxus consists of a 

 closed thick-walled tube lying along the dorsal side of the 

 body above the notochord. 



Viewed externally, it is a perfectly plain, more or less 

 cylinder-shaped structure, without any constrictions or 

 enlargements whatever. Its largest diameter in the adult 

 occurs about the middle of its course, and not at its 

 anterior end. 



Posteriorly it is nearly coextensive with the notochord, 

 and, like it, tapers down almost to a point.* Anteriorly it 

 terminates abruptly some distance behind the front end 

 of the notochord. (Cf. Figs. 3 and 11.) 



If the dorsal nerve-cord be removed from the body and 



* The extreme posterior end of the nerve-cord is usually swollen out into 

 a small ampulla-like dilatation. (PouCHET, RoHON, RETZIUS.) RETZIUS 

 has observed that occasionally the nerve-cord is prolonged beyond the dilata- 

 tion and actually bends round the posterior end of the notochord. 



