388 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



teleost fish. The blood from the single ventricle, v (dark 

 blood), is thrown into the aorta, which then divides into 

 three or four arches on each side, in order to pass through 

 the gill arches, to be distributed 

 in the gill fringes and aerated 

 there. From the gill fringes it 

 is gathered as bright blood, and 

 passes on to unite into the ab- 

 dominal aorta without going back 

 to the heart, and is then distrib- 

 uted to the tissues. Its course 

 is shown by the arrows. 



This is schematic. The ac- 

 tual course is shown in Fig. 268, 

 which shows also the change in 

 the blood in passing through 

 the gill fringes. As to variation 

 from this type, it is sufficient to 

 say that in sharks there are five 

 arches, in lampreys seven, and 

 in lancelets twenty or more on 

 each side. On the other hand, 

 the Dipnoi or lung fishes, lepido- 

 siren, and prptopterus, like the reptiles, have only three 

 on each side. 



Several interesting questions occur here. (1) We 

 have seen that fishes have a single heart. Which is it ? 

 Physiologically it is a pulmonic heart, for nothing but 

 dark blood visits it. But morphologically it doubtless 

 corresponds to the whole double heart of mammals. (2) 

 Is the circulation of reptiles or that of fishes the better? 

 Physiologically, in some respects the one, in some re- 

 spects the other is the better. The reptile has the ad- 

 vantage of air breathing, which aerates the blood more 

 efficiently than water breathing, but, on the other hand, 



Fig. 267. — Diagram showing 

 structure of the heart and 

 course of circulation in 

 fishes. 



