GENERAL CHARACTERS OF THE VERTEBRATA. 203 



mass. This is due to the presence in it of numerous micro- 

 scopical particles, which are known as the " blood-corpuscles," 

 the fluid itself being colorless. In Fig. 99 are represented 



FIG. 99. Blood-corpuscles, magnified, a Man ; & Goose; c Crocodile; d Frog; e Skate. 



some of the forms of blood-corpuscles which are found in dif- 

 ferent divisions of the Vertebrata. 



The blood is always distributed through the body by 

 means of a system of closed tubes, which constitute the "blood- 

 vessels," and, with the single exception of the Lancelet, it is 

 always propelled by means of a contractile muscular cavity or 

 " heart." The heart and other circulatory arrangements differ 

 considerably in different classes of the Vertebrata, but these 

 differences will be best considered at a later period. Respira- 

 tion in all the Vertebrata is effected by means of distinct 

 breathing-organs, assisted in many cases by the skin. In the 

 water-breathing Vertebrates, such as fishes, the respiratory 

 organs are in the form of gills or branchiae, which are richly 

 supplied with blood, and are exposed to the influence of water 

 holding oxygen in solution. In the air-breathing Vertebrates, 

 the breathing organs are in the form of lungs. These essen- 

 tially consist of cellular or spongy organs, placed in the cavity 

 of the chest, richly furnished with blood-vessels, and receiving 

 constant supplies of fresh air by means of a tube which opens 

 in the throat and is known as the " windpipe," or trachea. In 

 the higher Vertebrates the heart becomes a double organ, one 

 side being concerned wholly with driving the impure (venous) 

 blood to the lungs, while the other side propels the pure oxy- 

 genated (arterial] blood to all parts of the body. 



The waste substances of the body of which the most im- 

 portant are water, carbonic acid, and the peculiar substance 

 called urea are got rid of by the skin and lungs, but prin- 

 cipally by two glands which are called the kidneys. The ex- 

 cretion of urea from the body, as a general rule, is wholly 

 effected by means of the kidneys alone ; and this is their most 

 important function, as the retention of this substance within 

 the body rapidly causes death. The secretion of the kidneys 

 is sometimes got rid of by means of special canals appropriated 



