54 



ANATOMY OF AMPHIOXUS. 



ME* 



Fig. 25. View of portion 

 of sub-intestinalvein of Amphi- 

 oxus, to show its fenestrated 

 character in the posterior re- 

 gion. (After SCHNEIDER.) 



a. Anterior, p. Posterior. 



another, as shown in Fig. 25, and 

 so there is produced a fenestrated 

 structure in the vein. The hepatic 

 vein has a similar fenestrated char- 

 acter, and this was what was meant 

 by speaking of it above as being 

 "multiple." 



The sub-intestinal vein reappears 

 in the embryos of all the higher 

 fishes and Amphibia, where it breaks 

 up into capillaries in the liver. In 

 these forms, however, it does not 

 persist long as the main venous 

 trunk, but becomes replaced almost 

 entirely by the development of two 

 large veins, which arise on either 

 side of the dorsal aorta. These are 

 the so-called cardinal veins. The 

 sub-intestinal vein mostly disappears 

 after the formation of the cardinal 

 veins, but persists as a second-class 

 vessel in the lampreys and in some 

 sharks, lying, in the latter, in the 

 spiral valve of the intestine.* More- 

 over, its posterior portion, which 

 lies in the tail, persists as the caudal 

 vein. 



* The sub-intestinal vein is also persistent in 

 the following Urodele Amphibia Salaman- 

 dra, Triton, and Pleurodeles. (See F. HOCH- 

 STETTER. Beitrage zur vergleichenden Anatomie 

 tind Entwicklungsgeschichte des Venensystems 

 der Amphibien und Fische. Morph. Jahrb. 

 XIII. 1888. pp. 119-172.) 



