464 DEVELOPMENT OF ELASMOBRANCH FISHES. 



splanchnic vein. At the cloaca, where the gut enlarges and 

 comes in contact with the skin, this vein is compelled to bi- 

 furcate (PI. 1 8, fig. 6 d, v. cau.}, and usually the two branches 

 into which it divides are unequal in size. The two branches 

 meet again behind the cloaca and take their course ventral to 

 the postanal section of the gut, and terminate close to the end of 

 the tail, PI. 18, fig. 6 c, v. can. In the tail they form what is 

 usually known as the caudal vein. The venous system of Scyl- 

 lium or Pristiurus, during the early parts of stage K, presents 

 the simple constitution just described. 



Before proceeding to describe the subsequent changes which 

 take place in it, it appears to me 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, con- 

 nected 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 ex- 

 cept 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. u, fig. 9 b, 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- 



