234 VENOUS SYSTEM. 



Scyllium or Pristiurus, during the early parts of stage K, pre- 

 sents the simple constitution just described. 



Before proceeding to describe the subsequent changes which 

 take place in it, it appears to nae worth pointing out the re- 

 markable resemblance which the vascular system of an Elas- 

 mobranch presents at this stage to that of an ordinary Annelid 

 and Amphioxus. It consists, as does the circulatory system, in 

 Annelids, of a neural vessel (the aorta) and an intestinal vessel, 

 the blood flowing backwards in the latter and forwards in the 

 former. The two in Elasmobranchs communicate posteriorly by 

 a capillary system, and in front by the arterial arches, connected 

 like the similar vessels in Annelids with the branchiae. Striking 

 as is this resemblance, there is a still closer resemblance between 

 the circulation of the Scyllium embryo at stage K and that of 

 Amphioxus. The two systems are in fact identical except in 

 very small details. The subintestinal vessel, absent or only 

 represented by the caudal vein and in part by the ductus 

 venosus in higher Vertebrates and adult Fish, forms the main 

 and only posterior venous trunk of Amphioxus and the embryo 

 Scyllium. The only noteworthy point of difference between 

 Amphioxus and the embryo Scyllium is the presence of a portal 

 circulation in the former, absent at this stage in the latter ; but 

 even this is acquired in Scyllium before the close of stage K, 

 and does not therefore represent a real difference between the 

 two types. 



The cardinal veins make their appearance before the close 

 of stage K, and. very soon unite behind with the unpaired 

 section of the caudal vein (PI. xi. fig. 9 h, p. cav. and v.). On 

 this junction being effected retrogressive changes take place in 

 the original subintestinal vessel. It breaks up in front into a 

 number of smaller vessels ; the lesser of the two branches con- 

 necting it round the cloaca with the caudal vein first vanishes 

 (PL XI. fig. 9 a, v)y 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 

 posterior cardinals open into it some little distance in front of 

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

 disappearance of the postanal section of the gut the caudal 

 vein is made to appear as a superintestinal and not a sub- 



