CHAP. IX CIRCULATORY ORGANS 435 



(Fig. 1 10,'a. br. a). Each afferent artery passes to the corre- 

 sponding gill, and there branches out into smaller and 

 smaller arteries, which finally open into a network of delicate 

 capillaries (p. 95), with which the connective tissue of the 

 branchial filaments is permeated. The blood in these 

 respiratory capillaries is therefore brought into close relation 

 with the surrounding water, and as the blood flows through 

 them it exchanges its carbon dioxide for oxygen, obtained 

 from the air dissolved in the water. 



From the respiratory capillaries the blood is collected 

 into minute arteries which join into larger and larger 

 trunks, and finally unite into efferent branchial arteries" {e. 

 br. a) by which the purified blood is carried from the 

 gills. The efferent arteries of the right and left sides unite 

 in a median longitudinal artery, the dorsal aorta (d. ao), 

 which passes backwards, immediately beneath the vertebral 

 column, to the end of the tail. 



From the efferent branchial arteries and the dorsal aorta 

 are given off numerous arteries supplying the whole of the 

 body with blood. The most important of these are paired 

 carotid arteries (c. a) to the head, and subclavians (scl. a) to 

 the pectoral fins ; unpaired splanchnic arteries (cl.a, ms.a), to 

 the enteric canal, liver, pancreas, and spleen ; numerous 

 paired renals (r. a) to the kidneys, spermatic {sp. a) or 

 ovarian arteries to the gonads, and a pair of iliacs (//. a) to 

 the pelvic fins. The posterior part of the dorsal aorta, 

 supplying the tail, is contained in the hsemal canal of the 

 caudal vertebrae, and is often spoken of as the caudal artery 

 {cd. a). 



The arrangement of the arteries in the tadpole (p. 206) is very similar 

 to that described above, and the diagram (Fig. no) would serve almost 

 equally well for a tadpole as for a fish. In the former there are four 

 pairs of aferent and efferent branchial arteries (corresponding to the 



F F* 



