INTERNAL ANATOMY. 55 



The same vessel, therefore, which constitutes the main 

 venous trunk of the adult Amphioxus performs the same 

 function in the evibryos of the higher fishes. We can thus 

 deduce a good deal of evidence from a consideration of the 

 vascular system alone, pointing to the primitive and ances- 

 tral character of Amphioxus. 



If we compare broadly the vascular system of Amphioxus 

 with that of a segmented worm like the common earth- 

 worm, we are at once confronted with certain obvious 

 superficial resemblances. Here, as in Amphioxus, the 

 vascular system comprises two main longitudinal trunks, 

 one lying above the intestine and the other below it, and 

 furthermore, they are connected together at intervals by 

 circular vessels which form complete rings round the 

 alimentary canal in the same way as do the vessels which 

 pass through the pharyngeal bars of Amphioxus. 



It is only when we come to enquire into the direction 

 of flow of the blood in the two cases that we meet with a 

 striking contrast between them. Whereas in Amphioxus 

 the blood flows in the dorsal aorta from before backwards 

 (see Fig. 20), and in the sub-intestinal vein together with 

 the branchial artery, from behind forwards, in the worm, on 

 the contrary, these directions are reversed, and the blood 

 flows from behind forwards in the dorsal vessel, and from 

 before backwards in the ventral vessel. 



The Excretory System. 



The excretory function is so intimately bound up with 

 the circulation that a description of the organs which 

 serve this function follows naturally after the consideration 

 of the vascular system. The apparent absence of definite 

 excretory organs in Amphioxus was for a long time one of 

 the greatest difficulties in the way of a correct appreciation 



