144 



ZOOLOGY 



SECT. 



on the ventral aspect of the body, in front of the pectoral arch, and 

 between the two series of branchial pouches. Its dorsal wall is 

 supported by the basi -branchial cartilage. Placing it in communi- 

 cation with the abdominal cavity is a canal the pericardio-peri- 

 toneal canal. The heart (Fig. 772) consists of four chambers sinus 

 venosus (sin. ven.), auricle (aur.), ventricle (vent.), and conus arteriosus 

 (con.}, through which the blood passes in the order given. The sinus 

 renosus is a thin-walled, transverse, tubular chamber, into the ends 

 of which the great veins open. It opens into the auricle by an 

 aperture, the sinu-auricular aperture. The auricle is a large, tri- 

 angular, thin-walled chamber, situated in front of the sinus veno- 

 sus and dorsal to the ventricle. Its apex is directed forwards, and 

 its lateral angles project at the sides of the ventricle : it commu- 

 nicates 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 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. 774) gives 

 origin to a series of paired afferent branchial 

 arteries (br.v.), 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 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 of 

 its side. In Chiloscyllium (Fig. 774) the arrangement is some- 

 what different. 



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

 arteries. These efferent vessels 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 run inwards and backwards from the loops under 

 cover of the mucous membrane of the roof of the mouth to unite 

 in a large median trunk the dorsal aorta. From the first efferent 

 vessel, that from the first or hyoidean gill, arises the carotid 

 artery, which runs forwards and bifurcates to form the internal 

 and external carotid arteries, supplying the head with arterial 

 blood. A hyoidean artery arises further out from the same vessel, 



Fin. 773. --Chiloscyllium. 



Branchial sac exposed from 

 the outside. 



