EVOLUTION OF THE VERTEBRATE 183 



admitting water into its mouth and then forcing it past 

 the gills to escape at the gill-slits ; the capillaries in the 

 mucous membrane of the laminae of the gills absorbing 

 oxygen from the water as it passes through, and giving up 

 carbonic acid. 



The Fish's heart drives the venous blood of the general 

 circulation through the branchial arteries (a in figure) to 

 the gill capillaries, where, having got rid of its carbonic 

 acid, it absorbs oxygen and returns through the branchial 

 veins (6) to the aorta. The branchial circulation is thus 

 to all purposes composed of a number of vascular rings, 

 usually four, each formed by the junction of a right and 



Fig. 56. — Diagram of the branchial circulation of the typical 

 Fish. The venous part of the system is shaded, including the 

 arterial bulb. The capillaries would link up the shaded with 

 the unshaded half of each ring at x. 



left arching bloodvessel, the points of junction being the 

 arterial bulb and the aorta (Fig. 56). 



Consistently with our theory that the Fish has evolved 

 from a primitive " serio-medusoid " organism, we recognise 

 in the branchial rings ancestral medusoid canal repetition ; 

 each ring representing, as it were, the circular canal which 

 runs round the rim of the medusoid bell. Derivatively, the 

 rings are in the Fish what the loop-hearts are in the Worm 

 (page 117), though the Vertebrate and the Invertebrate have 

 evolved on quite separate paths from their primitive serio- 



