G5 



the blood returning to the ventricle. This vessel passes forwards along the dorsal 

 edge of the jugular bone, and beneath the basi-branchials it divides, giving off on each 

 side an artery to each of the first four branchial arches ; from the artery of the first 

 arch a branch passes to the pseudobranchia. The fifth arch, bearing no branchial 

 filament, does not receive a special branchial artery. The branchial arteries run along 

 the branchial arches giving off smaller vessels which break up into the capillaries of 

 the branchial filaments. These capillaries unite again into the efferent branchial 

 vessels which run in the branchial arches towards the dorsal side of the pharynx or 

 throat, where they unite together into a single median dorsal aorta. From the efferent 

 branchial vessels are given off arteries which supply the brain, skull, and other parts 

 of the head. From tlie dorsal aorta is given off at the anterior end of the body cavity 

 the coeliac artery which supplies the intestine spleen, and liver with arterial blood. 

 The dorsal aorta runs backwards in the canal formed by the series of divergent bases 

 of the ventral spines of the vertebrae, and is therefore in its anterior part dorsal to the 

 kidneys. It gives off special arteries which descend to supply the generative organs. 

 It also throughout its course gives off a pair of arteries to each segment of the lateral 

 muscles, fi-om which branches supply the skin, bones, and all parts of the body. 



The return of the exhausted venous blood from the tissues to the heart takes place 

 by a somewhat curious route. The veins of the head unite into the two anterior 

 cardinals, already mentioned, one on each side. But nearly all the veins of the trunk 

 fall ultimately into the caudal vein which runs forwards in- the same bone-protected 

 canal as the aorta, as far as the interval between the ventral spines of the ninth and 

 tenth vertebra3, where it bends suddenly towards the ventral edge of the body and 

 enters the dorsal side of the renal mass. The veins of the trunk in front of the caudal 

 vein fall into the kidney by separate collecting veins. A number of the veins of the 

 ventral side of the trunk, behind the kidney, unite into a longitudinal vein, which runs 

 forwards along the left side of the median skeletal partition and enters the posterior 

 apex of the part of the kidney which extends backwards on the left side behind tlie 

 main body cavity. Thus nearly aU the venous blood in the trunk is conveyed into the 

 renal mass, where the urinary excretion is extracted from it, and whence it is passed 

 on by the posterior cardinal veins to the heart. The veins of the spleen and intestines on 

 the other hand unite into a single vein which enters the liver where it breaks up into 

 capillaries, and these unite again to form the hepatic vein which opens into the auricle. 



K 



