GENERAL CHARACTERS OF THE VERTEBRATA. 223 
absorbed by the veins which ramify on the intestinal canal, and 
which ultimately unite to form a great vessel, called the “ vena 
porte,” which goes to the liver. The materials, however, which 
are taken up in this way also ultimately reach the circulating 
blood. In this way, therefore, fresh matter is being constantly 
added to the blood to replace the waste caused by the perform- 
ance of the vital functions. 
The blood is thus formed out $e the materials which are 
taken into the alimentary canal as food ; and in all the Verte- 
brata (with one exception) it is of a red colour, when viewed in 
mass, ‘This is due to the presence in it of numerous microsco- 
pical particles, which are known as the “ blood-corpuscles,” the 
fluid itself being colourless. In fig. 121 are represented some 
of the forms of blood-corpuscles which are found in different 
divisions of the Vertebrata. 
Fig. 121.—Blood-corpuscles, magnified. a Man; & Goose; ¢ Crocodile ; 
@ Frog; e Skate. 
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 pro- 
pelled by means of a contractile muscular cavity or “ heart.” 
The heart and other circulatory arrangements differ consider- 
ably in different classes of the Vertebrata, but these differences 
will be best considered at a later period. esfzrazion 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 gz//s or branchize, 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 Zuxgs, These essentially consist of cellular | 
or spongy organs, placed in the cavity of the chest, richly fur- 
nished 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 “ wind-pipe,” or ¢rachea. Inthe higher Vertebrates 
the heart becomes a double organ, one side being concerned 
