74 ANATOMY OF AM PHI OX US. 



intestinal vein, and in the young larva the two vessels are 

 continuous. The direct continuity is subsequently inter- 

 rupted by the development of the hepatic ccecura, and the 

 consequent insertion of a capillary portal system into the 

 circulation. In the Selachian embryo, a series of similar 

 vessels, six in number, connecting the dorsal aorta with 

 the sub-intestinal vein, have been shown to be in close cor- 

 respondence with the pronephric tubules, and to form at 

 the level of the tubules a series of rudimentary glomer- 

 uli (Figs. 35 A and B)? 



Such resemblances as the above are demonstrative, and 

 are sufficient to prove that the excretory tubules of Am- 

 phioxus belong to the pronephric system, and that in this 

 respect, also, the adult Amphioxus presents features which 

 are characteristic of the embryos, or larvae, of the higher 

 forms. 



Although convinced as to the essential identity of the 

 excretory tubules of Amphioxus with the pronephros of 

 the craniate Vertebrates, it must be remembered that 

 there is one apparently great difference between them. 

 Whereas in Amphioxus the pronephros (applying this 

 term to the tubules considered collectively) occurs in the 

 region of the perforated pharynx, in all the higher Verte- 

 brates it occurs behind the pharynx, and is quite absent 

 from the regies of the gill-slits. This difference, however, 

 which might at first sight appear serious, is, in reality, 

 most instructive. As Boveri points out, it shows almost 

 conclusively that the pharynx of Amphioxus does not 

 correspond to the pharynx alone of the higher forms, but 

 to the pharynx together with the anterior portion of the 

 alimentary canal. 



In the Craniota the gill-clefts, which are present in a 

 limited number, have becomfc involved in the complicated 



