PHYLUM CHORDATA 407 



The smallest blood vessels are called capillaries. The ex- 

 change of substances between the blood and tissues takes place 

 through the walls of the capillaries. Certain capillaries unite 

 to form veins, which carry blood from all parts of the body to the 

 heart. Arterial blood leaves the heart chiefly through the aorta. 

 The aorta gives off branches which in turn branch until they end 

 in minute arterial capillaries. The functions of the circulatory 

 system are like those of this system in invertebrates, i.e. the 

 transportation of nutriment, oxygen, and waste products from 

 one part of the body to another. In close connection with the 

 circulatory system are a number of spaces and channels com- 

 prising the lymphatic system. Lymph is a clear fluid containing 

 ameboid cells like the colorless blood corpuscles. 



Respiratory System. - - Two kinds of respiration may be 

 recognized, (i) external respiration, during which oxygen passes 

 into the blood from the air or water and carbon dioxide passes 

 out of the blood, and (2) internal respiration, during which the 

 blood supplies oxygen to and takes carbon dioxide from the 

 cells of the body. External respiration is carried on by gills in 

 most aquatic vertebrates and by lungs in terrestrial vertebrates. 

 Respiration also takes place to some extent through the skin. 

 Oxygen unites readily with the haemoglobin in the red corpuscles. 

 The haemoglobin is then transported by the blood from the 

 respiratory organs to the capillaries, where it breaks up, the 

 oxygen being absorbed by the tissues. Carbon dioxide from the 

 tissues becomes chemically combined with the sodium in the blood, 

 is carried to the respiratory organs, and discharged to the outside. 



Excretory System. - - The substances resulting from the oxi- 

 dation of protoplasm are eliminated by the kidneys, respiratory 

 organs, and skin. These waste products are carried by the blood. 

 Carbon dioxide is eliminated by the respiratory organs. Ni- 

 trogenous waste products are excreted by the kidneys in the form 

 of urea or uric acid. Ducts, called ureters, lead from the kidneys 

 either directly to the outside or empty the excretion into a 

 storage vesicle, the urinary bladder. 



