XIII 



PHYLUM CHORDATA 



155 



forming the most conspicuous part of tlu> heart when looked at 

 from the ventral surface. From it the conns 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 \alves, anterior and posterior, i\\v former 

 consisting of three, the latter of three oi' four. The ventral aorta 

 (Fig. 811)) gives origin to a scries of jjaired afferent h-anehial 

 arteries {af. hr.), one for each branchial pouch. In Scyllium 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 forwaids a little distance and bifurcates to f(»nu 



</.c 



Fig. S1!>.— The heart and branchial arteries of Scyllium, from the side. af. '</•.!— •', afferent 

 branchial arteries : av. auricle ; r. a. conns arteriosus ; rlj—^, branchial clefts ; ror. coronaiy 

 artery ; <L ao. dorsal aorta ; il. r. dorsal carotid artery ; (;/. ir.i— 9, efferent braiichiril 

 arteries; (y). 6;-. i—-i, epibranchial arteries ; /»/!. mandibular artery ; .«7). spiracle ; n.H. snlv 

 claviau artery ; .f. i: sinus venosus ; r. ventricle ; r. ao. ventral aorta ; r. c. ventral carotid 

 artery. (From Parker's Pructkal Zooloyy.) 



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

 bifurcates to form the first and second afferent vessels {af. Ir^., 

 af. Irr) of its side. In Hemiscyllium (Fig. 820) the arrangement 

 is somewhat different. 



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

 arteries. These efferent vessels (Fig. 819 {ef. hr.)) 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. hr.) 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 aoo'ta (d. ao.). A dorsal carotid artery (d. c.) is 

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



