xm PHYLUM CHORDATA 155 



forming the most conspicuous part of the heart when looked at 

 from the ventral surface. From it the comis arteriosus runs 

 forwards as a median stout tube to the anterior end of the peri- 

 cardial cavity, where it gives off the ventral aorta. It contains 

 two transverse rows of valves, anterior and posterior, the former 

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

 (Fig. ^ 819) 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 



Fig. 819.— The heart and branchial arteries of Scyllium, from the side. af. («-.l— 5, afferent 

 branchial arteries ; au. auricle ; c. a. eonus arteriosus ; c^i — 5, branchial clefts ; cm\ coronary 

 artery ; d. ao. dorsal aorta ; d. c. dorsal carotid artery ; cf. hr.^ — ^, efferent branchial 

 arteries; eji. ^jr.l—**, epibranchial arteries ; ?»«. mandibular artery ; .syj. spiracle ; a. cl. sub- 

 claviaii artery ; s. r. sinus venosus ; t. ventricle ; v, ao. ventral aorta ; v. c. ventral carotid 

 artery. (From Parker's Practi-^al Zoology.) 



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

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

 af. br.'^) of its side. In Hemiscyllium (Fig. 820) the arrangement 

 is somewhat different. 



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

 arteries. These efferent vessels (Fig. 819 (cf. 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 (hyoidean) 



