MORPHOLOGY OF ANIMALS 



portance in explaining the structure of the circulatory 

 organs in air-breathing vertebrates. In annelids, crusta- 

 ceans, and mollusks the blood flows directly from the heart 

 to all parts of the body, whence it is gathered into trunks 

 which carry it to the gills; from these organs it is then 

 returned oxygenated to the heart (Fig. 13, A, B). In 

 fishes the blood passes from the heart directly to the gills, 

 whence it is gathered into the dorsal aorta and distributed 

 to all parts of the body; it is then returned laden with 

 waste products from the tissues to the heart (Fig. 16, A). 



hr 



Fio. 16. Diagrams of the Course of Circulation in different classes of Vertebrates. A, Fish, 

 blood goes from the heart {h) to the gills {br), thence to the swim bladder (/) and body {k) 

 and then back to the heart. B, Frog, blood goes from the heart (^h) through 3rd and 4th aortic 

 arches to the body {k) and then back to the heart (A) ; through the 6th aortic arch to the 

 lungs and then back to the heart. C, Birds and Mammals, blood goes from left side of heart 

 {hk) through the 3rd and 4th aortic arches to the body {k) and then back to the right side of 

 the heart, whence it goes to the lungs (/) and then back to the left side of the heart. (After 

 Hatschek). 



The heart of fishes consists of one auricle and one ven- 

 tricle ; it is essentially a simple tube more or less bent upon 



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