THE VENOUS SYSTEM. 465 



necting it round the cloaca with the caudal vein first vanishes 

 (PL 11, fig. 9 a, v), and then the larger; and the two cardinals 

 are left as the sole forward continuations of the caudal vein. 

 This latter then becomes prolonged forwards, and the two pos- 

 terior cardinals open into it some little distance in front of the 

 hind end of the kidneys. By these changes and by the dis- 

 appearance of the postanal section of the gut the caudal vein is 

 made to appear as a superintestinal and not a subintestinal 

 vessel, and as the direct posterior continuation of the cardinal 

 veins. Embryology proves however that the caudal vein is a 

 true subintestinal vessel 1 , and that its connection with the car- 

 dinals is entirely secondary. 



The invariably late appearance of the cardinal veins in the 

 embryo and their absence in Amphioxus leads me to regard 

 them as additions to the circulatory system which appeared 

 in the Vertebrata themselves, and were not inherited from their 

 ancestors. It would no doubt be easy to point to vessels in 

 existing Annelids which might be regarded as their equivalent, 

 but to do so would be in my opinion to follow an entirely false 

 morphological scent. 



The circulation of the yolk-sack. 



The observations recorded on this subject are so far as I 

 am acquainted with them very imperfect, and in most cases the 

 arteries and veins appear to have been transposed. 



Professor Wyman 2 , however, gives a short description of the 

 circulation in Raja Batis, in which he rightly identifies the 

 arteries, though he regards the arterial ring which surrounds the 

 vascular area as equivalent to the venous sinus terminalis of the 

 Bird. 



The general features of the circulation are clearly portrayed 

 in the somewhat diagrammatic figures on PI. 9, in which the 

 arteries are represented red, and the veins blue 3 . 



1 The morphological importance of this point is considerable. It proves, for 

 instance, that the haemal arches of the vertebrae in the tail (vide pp. 373 and 374) 

 potentially, at any rate, encircle the gut and enclose the body-cavity as completely as 

 the ribs which meet in the median ventral line may be said to do anteriorly. 



2 Memoirs of the American Academy of Arts and Sciences, Vol. IX. 



3 I may state that my determinations of the arrangement of the circulation were 

 made by actual observation of the flow of the blood under the microscope. 



