206 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



into the subcardinal veins and through the kidneys into the posterior cardinal veins. There 

 next occurs a break between the anterior and posterior parts of the posterior cardinal veins. 

 The anterior parts form a connection with the subcardinal veins. The posterior parts retain 

 their connection with the caudal vein. There is then a reversal of flow through the kidneys, 

 since the blood now passes from the caudal vein into the posterior parts of the posterior cardinal 

 veins, now called the renal portal veins, through the kidneys into the subcardinal veins, and 

 from them into the anterior portions of the posterior cardinal veins (Fig. 556). At first the 

 channels through the kidneys are direct connections between the renal portal and subcardinal 

 veins, but later they break up into a capillary network. In this way the renal portal sys- 

 tem is established. The blood from the tail must pass through a capillary network in the 

 kidneys. 



We have now carried the circulatory system up to the stage of development in which it 

 occurs in adult elasmobranchs. The further evolution of the circulatory system will best be 

 followed on the specimens. 



6. The segmental character of the circulatory system. Like most systems of vertebrates, 

 the circulatory system exhibits marked traces of an originally highly segmented condition. 



mouth __ 1 -+ __ caudal artery 



caudal vein 

 anus 

 aortkarch vent ral aorta hca ^ t vitelline vein abdominal vein 



sUbmtestinal vein 



FIG. 56. Diagram of the hypothetical primitive vertebrate circulation from which that of present 

 vertebrates was probably derived, emphasizing the markedly segmental arrangement of the primitive 

 vessels. (After Kingsley's Comparative Anatomy of Vertebrates, copyright by P. Blakiston's Son and 

 Company.) 



However, even in vertebrate embryos the segmental arrangement of the blood vessels is some- 

 what imperfect. It is helpful to an understanding of the cirulatory system to imagine that it 

 is derived from a highly segmented condition such as that present in annelids. In this hypo- 

 thetical ancestral state, as illustrated in Figure 56, there are three main longitudinal vessels: 

 a dorsal somatic vessel, corresponding to the dorsal aorta; a ventral somatic vessel, correspond- 

 ing to the abdominal vein; and a splanchnic vessel, corresponding to the subintestinal and 

 vitelline veins, and the ventral aorta. In each segment these longitudinal vessels are connected 

 by loops, which are of two kinds, somatic and splanchnic. In the anterior part of the body 

 only the splanchnic connecting loops are present, represented by the aortic arches connecting 

 the dorsal and ventral aortae. Posterior to the heart each segment is provided with a somatic 

 and a splanchnic loop, the former passing in the body wall from the aorta to the ventral abdomi- 

 nal vein, and the latter passing in the intestinal wall from the aorta to the subintestinal vein. 

 These transverse loops pictured as continuous are in reality interrupted by capillaries in the 

 body and intestinal walls. In vertebrates all of the connecting loops are lost except the aortic 

 arches; but segmentally arranged transverse vessels corresponding to the loops are plainly 

 evident in vertebrate embryos, and persist in certain regions of the adult, as in Figure 54. 

 For more extensive accounts of the comparative anatomy of the circulatory system, K or 

 VV should be consulted. 



