36 ELEMENTS OF COMPARATIVE ZOOLOGY. 



an arterial cone or an arterial bulb. These differ in this: 

 the arterial cone is really an outgrowth of the heart, and 

 contains, on its interior, valves to prevent the flow of the 

 blood back into the ventricle; the arterial bulb, on the 

 other hand, is merely a muscular thickening of the ventral 

 aorta, and contains no valves. 



The blood, returning to the heart, bears with it the waste 

 from all parts of the body, and prominent among these is 



FIG. 14. Types of Fish-hearts, a, auricle ; fc, bnlbus ; c, conus ; v, ventricle. 



carbonic dioxide; in short, it is what physiologists call 

 venous blood. This is forced forward, through the ventral 

 aorta and the branchial arteries, to the gills. Through the 

 thin walls of these it comes in close connection with the 

 water, and the carbonic dioxide is given off, while oxygen, 

 from the air dissolved in water, is taken into the blood, 

 which thus becomes arterial blood, and is distributed to all 

 parts of the system through the dorsal aorta and other 



It is interesting to note why a fish dies when taken from 

 the water. It is simply because it cannot obtain air enough. 

 When the fish is in the water the gills are floated out so 

 that all parts of them are exposed to the stream passing 



