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body by a series of efferent branchiate or epibranchials, 

 which join to form two longitudinal vessels, the roots of the 

 dorsal aorta. In Amniota, in which the gill-clefts are not 

 respiratory, the afferents and efferents are directly con- 

 tinuous aortic arches, which are reduced in number. For- 

 ward extensions of the dorsal aorta roots and the ventral 

 aorta, into the head region, form respectively the internal 

 and external carotid arteries. 



The blood returns from the head region in anterior 

 cardinal veins, and from the body in posterior cardinal 

 veins. The anterior and posterior cardinal of each side 

 join to form a short transverse duct of Cuvier (precaval 

 sinus) ; these ducts enter the sinus venosus, one at each 

 side. The veins from the stomach, intestine, pancreas and 

 spleen join to form the hepatic portal vein, which enters 

 the liver and there branches into numerous capillaries. 

 The liver receives sugar from the blood, and stores it to 

 form glycogen. The blood then passes into the hepatic 

 veins which lead from the liver into the sinus venosus. 

 This circulation of venous blood through the liver, before 

 returning to the heart, is the hepatic portal system. The 

 median caudal vein from the tail region branches into two 

 renal portal veins, which enter the kidneys. After filtering 

 through the kidneys (renal portal system), the blood passes 

 by renal veins into the posterior cardinals. Birds and Mam- 

 mals are without a renal portal system (except in embryo). 

 The front portions of the renal portals were originally parts 

 of the posterior cardinals ; and the caudal and hepatic 

 portal veins represent the subintestinal vein. In the 

 higher Craniata, the ducts of Cuvier become the superior 

 venae cavae, and the posterior cardinals are replaced by 

 a single inferior vena cava. 



The veins have thinner walls than the arteries, and are 

 darker in colour, owing to the bluish tint of the deoxidised 

 blood. Some of the veins may form spacious sinuses 

 (e.g.. in Elasmobranch fishes), which are, however, never 

 ill-defined or without walls as in many Invertebrates. 



How is the Blood formed ? Mention its Constituents, and 

 explain its relation to the associated Lymphatic 

 System. 



The blood (corpuscles and plasma) is formed from meson- 

 chyme, and from cells derived from the vessel walls (i.e., 

 from discrete mesoderm). The corpuscles are developed 

 chiefly in the lymphatic " glands " and the spleen. The 



