GENERAL CHARACTEHS OF THE VERTEBKATA. 225 



The materials, however, which ai-e taken up iu this way also ulti- 

 mately reach the circulating blood. In this way, therefore, fresh 

 matter is being constantly added to the blood to replace the waste 

 caused by the performance of the vital functions. 



The blood is thus formed out of the materials which are taken 

 into the alimentary canal as food ; and in almost all of the Vei-fe- 

 brata it is of a red colour, when viewed in mass. This is due to 

 the presence in it of numerous microscopical particles, which are 

 known as the "blood-corpuscles,"' the fluid itself being colourless. 

 In fig. 163 are represented some of the forms of blood-corpuscles 

 which are found in different divisions of the Vertehrata. 



\.i 



I) 



b 



Fig. 163. — Blood-corpuscles, magnified, a Man ; & Goose ; c Crocodile ; 

 d Frog ; e Skate. 



The blood is always distributed through the body by means of a 

 system of closed tubes, which constitute the " blood-vessel.s," 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 diifer considerably in different 

 classes of the Vertehrata, but these differences will be best considered 

 at a later period. Respiration in all the Vertehrata is effected by 

 means of distinct breathing-organs, assisted in many cases by the 

 skin. In the water-breathing Vertebrates, such as fishes, the res- 

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

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

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

 breathing-organs are in the form of hmgs. These essentially 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, whilst the other side 

 propels the pure oxygenated (arterial) blood to all pai-ts of the body. 



The waste substances of the body — of which the most important 

 are water, carbonic acid, and the peculiar substance called V/rea — are 

 got rid of by the skin and lungs, but principally by two glands which 

 are called the kidneys. The excretion of urea from the body, as a gen- 

 eral rule, is wholly effected by means of the kidneys alone ; and this is 



