374 MANUAL OF ZOOLOGY , SECT, xn 



ventricle (v.\ and conus arteriosus (c. art^,. through which the 

 blood passes in the order given. The sinus venosit$v$> a thin- 

 walled, transverse, tubular chamber, into the ends of which 

 the great veins open. It communicates with the auricle by 

 an aperture, the sinuauricuhir aperture. The auricle is a 

 large, triangular, thin-walled chamber, situated in front of 

 the sinus venosus and dorsal to the ventricle. Its apex is 

 directed forwards, and its lateral angles project at the sides of 

 the ventricle : it communicates with the ventricle by a slit-like 

 aperture guarded by a two-lipped valve. The ventricle is a 

 thick-walled, globular chamber, forming the most conspicuous 

 part of the heart when looked at from the ventral surface. 

 From it the conus arteriosus runs forwards as a median stout 

 tube to the anterior end of the pericardial 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 valves. The ventral aorta 

 (v. ao.) gives origin to a series of paired afferent branchial 

 arteries (a. br. .), one for each branchial pouch. 



The blood passes from the gills by means of the efferent 

 branchial arteries (Fig. 21 1, . br. a.). These efferent vessels 

 form a series of loops, one running around the margin of 

 each of the first four internal branchial apertures : a single 

 vessel runs along the interior border of the fifth branchial 

 cleft and opens into the fourth loop. The four main efferent 

 branchial vessels 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*). From the first efferent vessel that from the first 

 or hyoidean gill, arises the carotid artery (c. #.), which runs 

 forwards and bifurcates to form the internal and external 

 carotid arteries, supplying the head with arterial blood. 



The dorsal aorta runs backwards throughout the length 



