CIRCULATORY SYSTEM 179 



ends, and open, as shown in figs. 41 and 42, 

 into the large dorsal coelomic ^canals above the 

 suspensory folds, 

 c. A series of coelomic spaces surround^he reproductive 

 organs. 

 4. The metapleural canals are a pair of wide spaces run- 

 ning along the lateral or metapleural folds. They 

 are possibly of a lymphatic nature. 



T. 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 contain^ but few corpuscles. 

 The principal vessels are as follows. 



1. The cardiac aorta is amedian 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 giU-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 horizontal 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 aortse are a pair of longitudinal vessels into 

 which the aortic arches of both primary and second- 

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



N 2 



