FISHES. 



321 



of the ventral aorta, and contains no valves. After pass- 

 ing through the afferent and efferent branchial arteries 

 (p. 311) the blood is collected in the dorsal aorta and 

 thence distributed to the body. 



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

 waste from all parts of the body, and prominent among 

 this is carbonic dioxide; in short, it is what physiologists 



FIG. 121. Types of Fish-hearts, a, auricle; b, bulbus; c, conus; v, ventricle. 



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 connec- 

 tion 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 vessels. Hence, as will readily be under- 

 stood, the heart of the fishes, in contrast to that of all 

 other vertebrates, receives only venous blood. 



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

 the water. It is simply because it cannot obtain air 

 enough. AVhen the fish is in the water the gills are floated 

 out so that all parts of them are exposed to the stream 



