GENERAL CHAEACTEKS OF THE VEETEBEATA. 2''2.5 



The materials, however, which are taken up in 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 j)er-forniance 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 Vertc- 

 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-corpu.'-cles 

 which are found in different divisions of the Vertehrata . 



%- 



Fig. 163. — Blood-corpusolPS, magniticd. a JIan ; ?< GoDse ; c Crocodile ; 

 d Frog ; e Blititu. 



The blood is always distributed through the body )-iy means of a 

 system of closed tulies, which constitute the " blood-vessels," and 

 with the single exception of the Lancelet, it is always piropelled 

 by means of a contractile muscular cavity or " heart." The heart 

 and other circulatory arrangements difi'er considerably in different 

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

 at a later period. Respiration in all the Vertehrata is efl'ected by 

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

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

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

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

 ing oxygen in solution. In the air-lireathing Vertebr-ates, the 

 breathing-organs are in the form of lungs. These essentially consist 

 of cellular or spongy organs, placed in the cavity of the chest, richly 

 furnished with blood-vessels, and receiving con.stant supplies of fresh 

 air by means of a tube which opens in the throat and is known as 

 the "windpipe," or traehea. 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 parts of the body. 



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

 are loater, carbonic arid, and the peculiar substance called urea — 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 



