262 



CHARACTERS OF VERTEBRATE ANIMALS 



use, and the conditions are consequently comparatively simple, 

 the problem of separating two kinds of blood not having yet 

 arisen. The heart consists of a thin-walled venous sinus, which 

 receives the impure blood of the body and passes it on to a 

 single auricle, by which it is squeezed into a thick-walled 

 ventricle, continued again into a muscular tube, the arterial 

 cone. Valves to prevent the blood from running back the 

 wrong way are placed at the points of junction between sinus, 



1ST. DORSAL FIN 



MOUTH 



VENTRAL AORTA 



OONU8 ARTERIOSUS 



HEARTS AURICLE 



VENTRICLE 



SINUS VENOSUS 



Fig. 160. Side-dissection of Dog- Fish (Scyllium canicula]. Left half of liver has been removed 

 i, Internal opening of spiracular cleft. 2, 3, 4, 5, 6, Internal openings of gill-clefts. 



auricle, and ventricle, and several transverse rows of valves are 

 present in the arterial cone. From the ventral aorta, which 

 runs forward from the cone, five pairs of afferent gill-arteries 

 run out and carry impure blood to the gills, from which five 

 pairs of efferent gill-arteries conduct the purified blood, uniting 

 above to form the dorsal aorta that runs back to the end of 

 the tail, giving off numerous vessels by which the various organs 

 and regions receive their pure blood -supply. Each afferent 

 artery, with its corresponding efferent one, may be looked upon 

 as constituting an aortic arch (see p. 242). 



Breathing- organs (fig. 160). Upon the walls of the five 

 pairs of gill-slits, or pouches, to speak more correctly, numerous 

 closely-set gill-folds are placed, and in these folds there are 

 very numerous capillary blood-vessels, with which the gill- 

 arteries are in communication. Water is constantly taken in 



