1891.] The Evolution of the Circulatory Organs. 247 
Even after we enter into an examination of the vertebrates we 
will meet with a species, and, as we should be led to expect from 
an evolutionary standpoint, it happens to be of the very lowest 
class of the vertebrates, the Amphioxus, where the blood-fluid is 
colorless, and its form-elements are very small, indifferent cells. It 
is also significant that here also the lymphatic system is not 
entirely distinct from the vascular system. 
But in all other vertebrates, after we leave this lowest class, we 
shall find the two systems separate, and the blood color sed. 
While the blood is uniformly red, the form-elements of each of 
the great families of the vertebrates are distinctive and charac- 
teristic. The color of the blood now depends upon the coloring 
matter contained in the blood-cells, and not, as in the few instances 
of colored blood of the invertebrates, upon the colored plasma, , 
The blood-cells of all vertebrates are highly differentiated, and all 
contain a nucleus, save the red corpuscles of the highest, the 
mammals, and even here the nucleus is present in the foetal stages. 
The cells are generally flattened. In fishes, Batrachia, reptiles, and 
birds they are. oval and biconvex, while in mammals they are 
biconcave. The relative quantity of blood in the higher classes 
of the vertebrates remains the same, yet the relative cell, surface 
varies decidedly. 
The red blood-cells are essential to respiration and as carriers of 
oxygen to the tissues. Fishes consume very little oxygen, and 
so the red blood-cells are not relatively numerous, and they are 
called cold-blooded animals, having a temperature but little above 
that of the surrounding medium. The Batrachia are similarly 
constituted, but the reptiles have some higher qualites, but still 
inferior to birds and mammals, which are classed as warm-blooded. 
The physiological data contained herein are not the teachings 
of any special school of science, but the well-digested and gener- 
ally accepted conclusions of the principal modern authorities on 
comparative anatomy,—as may be seen more in detail in such 
works as Carpenter's “ Comparative Anatomy,” Cope’s “ Origin 
of the Fittest,” Gegenbaur’s “ Elements of Comparative Anatomy,” 
Huxley’s “ Anatomy of Vertebrated Animals,” and Owen's “ Com- 
parative Anatomy and Physiology.” 
