CIECULATOEY SYSTEM 177 



F. The Circulatory System. 



The main features of the circulatory system can be made 

 out by examination of a series of transverse sections. 



There is no heart, but the general course of the circulation 

 is the same as in other gill-breathing vertebrates. The blood 

 is colourless and contains but few corpuscles. 



The principal vessels are as follows. 



1 . The cardiac aorta is a median longitudinal vessel, running 



forwards along the ventral wall of the pharynx in the 

 floor of the hypobranchial groove. 



2. The aortic arches are a series of vessels, arising from the 



cardiac aorta, and running up the primary gill-arches. 

 Each aortic arch has at its ventral end a small con- 

 tractile dilatation, which lies in the fork between the 

 split ends of the axial rod of the gill-arch, the aortic 

 arch itself lying along the inner side of the rod. 



By means of vessels in the transverse bars of the 

 pharynx, the aortic arches of the primary arches are 

 connected with similar vessels in the secondary 

 arches. 



The most anterior aortic arch on the right side 

 is much larger than any of the others, and sends 

 branches forwards to the anterior end of the body. 



It is while the blood is in these vessels that it 

 becomes aerated by the respiratory stream of water 

 passing through the gill-slits. 



3. The dorsal aortae are a pair of longitudinal vessels into 



which the aortic arches of both primary and secondary 

 gill-arches open at their dorsal ends. They lie just 

 beneath the notochord, one on each side of the hyper- 

 . branchial groove, and project into the dorsal coelomic 

 spaces. 



The dorsal aortae carry the arteriali^ed blood back- 

 wards : they unite at the hinder end of the pharynx 

 to form a single vessel, which runs back along the 

 dorsal surface of the intestine, lying in the cceloniic 

 space surrounding it. 



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