VEETEBEATA. 19 



expelled again, right and left, through the gill-slits. 

 The blood, having become rich in oxygen in the gills, 

 is now once more fit for circulation through the body, 

 and therefore flows out of the gill-capillaries into 

 larger vessels, which finally unite into a single large 

 vessel that carries the purified blood to the various 

 parts of the body. In the arrangement of the heart 

 here described there is the disadvantageous condition 

 that the blood is obliged to traverse two sets of 

 capillaries (gill and body capillaries). This is not an 

 easy matter, for there is a great deal of friction 

 between the blood and the walls of the capillaries, 

 constituting a hindrance to its progress. The circula- 

 tion of the blood in fishes is consequently very slow, 

 and since the blood contains the oxygen which is 

 used by the various parts of the body, oxidation goes 

 on slowly in the body of a fish; hence the small 

 amount of heat developed there. Since fishes almost 

 immediately give off to their surroundings the small 

 amount of heat which they develop, they have no 

 constant body-temperature, varying in this respect 

 with the temperature of the surrounding water. Such 

 animals are termed cold-blooded. 



In all other Vertebrates a more rapid movement of 

 the blood is rendered possible by the insertion of a 

 second heart, quite similar in every respect to the 

 other heart, in the course of the blood between the 

 respiratory organs and the body. The first heart 

 drives the blood through the lung capillaries, and 

 therefore corresponds to the fish-heart j from these 

 capillaries the blood returns to the auricle of the 

 second heart, and from the ventricle of that heart 

 travels to the various parts of the body. When it has 

 completed this course, it returns to the auricle of the 

 first heart. Although these two structures work in- 

 dependently, they lie close together and make up 

 a single organ. We do not therefore speak of two 

 individual hearts, but of one heart with two halves. 



