A STUDY OF THE CIRCULATION OF BLOOD 155 



m 



ventral blood vessels carry blood to the capillaries in all 

 parts of the body. The earthworm, 

 then, has no heart, and it is there- 

 fore impossible to distinguish arteries 

 from veins. The blood is propelled 

 through the body by the rhythmical 

 contraction of the muscular walls of 

 the dorsal blood vessel and of the 

 five aortic arches described above. It 

 has been proved that the blood flows 

 through the dorsal vessel from the tail 

 toward the head end of the worm, 

 whence it is driven down to the ventral 

 vessel by the contraction of the aortic 

 arches. The course of the blood back 

 to the dorsal tube is still a matter of 

 dispute. 



Circulation in the Fish. The structure 

 of the fish heart is relatively simple. 

 It lies near the ventral surface and has 

 a single auricle and a single ventricle. 

 When the ventricle contracts, the 

 blood is forced forward a short distance 

 through an artery, which soon divides 

 into a series of branches on each side of 

 the body. By these arteries the blood 

 passes in a dorsal direction through the 

 gills, where it loses some wastes and 

 receives the oxygen that is dissolved in 

 the water. The gill arteries finally 

 empty into the dorsal aorta, from which 

 blood is distributed to all parts of the 

 body. From the systemic capillaries 

 it is brought back to the auricle by 

 veins of very large size. Hence the two systems of circula- 

 tion (called in man the pulmonary and systemic), instead of 



FIG. 60. Circulation 

 in the Fish. 



a, b = arteries to gills. 



c = ventricle. 



d auricle. 



e = opening of veins 

 into auricle. 



/= portal veins. 



g = intestine. 



h = vena cava. 



k = abdominal aorta. 



I = kidneys. 

 m = aorta which sup- 

 plies region of 

 tail. 



