xni PHYLUM CHORD ATA 151 



consisting of three, the latter of three or four. The ventral aorta 

 (Fig. 829) gives origin to a series of paired afferent branchial 

 arteries (af. br.), one for each branchial pouch. In Scy Ilium the 

 two most posterior arise close together near the beginning of the 

 ventral aorta, the third pair a little further forwards. The ventral 

 aorta then runs forwards a little distance and bifurcates to form 

 the two innominate arteries, right and left, each of which in turn 

 bifurcates to form the first and second afferent vessels (af. br 1 ., 

 af. br. z ) of its side. In Hemiscyllium (Fig. 830) the arrangement 

 is somewhat different. 



From the gills the blood passes by means of the efferent branchial 

 arteries. These efferent vessels (Fig. 829, ef. br.} form a series of 

 loops, one running around the margin of each of the first four 

 internal branchial clefts : a single vessel runs along the anterior 

 border of the fifth branchial cleft and opens into the fourth loop. 

 The four main efferent branchial vessels (epibranchials, ep. br.) run 

 inwards and backwards from the loops under cover of the mucous 

 membrane of the roof of the pharynx to unite in a large median 

 trunk the dorsal aorta (d. ao.). A dorsal carotid artery (d. c.) is 

 given off from the first efferent branchial. A branch (hyoidcan) 

 given off from the same efferent vessel supplies the pseudobranch, 

 and the blood from the latter is taken up by the ventral carotid 

 (v. c.). Both carotids run forwards to supply the head. 



The dorsal aorta (Fig. 829, d. ao.) runs backwards throughout 

 the length of the body-cavity, giving off numerous branches, and 

 is continued as the caudal artery, which runs in the canal enclosed 

 by the inferior arches of the caudal vertebrae. The first pair of 

 branches are the subclavians (s. cl.), for the supply of the pectoral 

 fins ; these are given off between the third and fourth pairs of 

 epibranchial arteries. The next large branch is the unpaired cosliac 

 (Fig. 827, ccel.) : this runs in the mesentery and divides into 

 branches for the supply of the stomach and liver, the first part of 

 the intestine, and the pancreas. The anterior mesenteric artery, 

 also median, supplies the rest of the intestine and gives off branches 

 to the reproductive organs. The lienogastric supplies part of the 

 stomach, the spleen, and part of the pancreas. The posterior 

 mesenteric is a small vessel mainly supplying the rectal gland. 

 Small renal arteries carry a small quantity of arterial blood to the 

 kidneys, and a pair of iliac arteries, likewise of small size, pass to 

 the pelvic fins. In addition to these a number of small arteries, 

 the parietal, supplying the wall of the body, are given off throughout 

 the length of the aorta. 



The veins are very thin- walled, and the larger trunks are remark- 

 able for their dilated character, from which they have obtained 

 the name of sinuses, though they are true vessels and not sinuses 

 in the sense in which the word is used in dealing with the Inverte- 

 brates (cf. p. 90). 



