224 VERTEBRATE ANIMALS. 
wholly with driving the impure (vexous) blood to the lungs, 
whilst the other side propels the pure oxygenated (arteria/) 
blood to all parts of the body. 
The waste substances of the body—of which the most impor- 
tant are water, carbonic acid, and the peculiar substance called 
wrea—are got rid of by the skin and lungs, but principally by 
two glands which are called the #zdueys. The excretion 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 to this alone; but in the 
lower Vertebrata it is discharged into the hinder extremity of 
the alimentary canal, and is evacuated along with the undigested 
portions of the food. 
The zervous system varies greatly in its development in the 
Vertebrata. In the little fish called the Lancelet, the main 
mass of the nervous system consists of a cord of nervous matter, 
representing the spinal marrow, but not having in front any 
enlargement which represents the brain. In all the other Ver- 
tebvata the central masses of the nervous system (termed the 
cerebro-spinal axis) consist of a nervous cord (the spinal cord) 
contained in the canal formed by the ewza/ arches of the verte- 
bree, and of an anterior mass of nervous matter; which is pro- 
tected by the skull, and is termed the “cerebrum,” or dvazz. The 
size and development, however, of the brain vary enormously in 
ditferent Vertebrates ; and in the lower forms the brain is little 
more than an aggregation or collection of nervous masses or 
“ganglia,” which are connected with the special senses, sight, 
hearing, taste, and smell, spectal organs for which are present 
in almost all the Vertebrata. 
Reproduction in the Vertebrata is always truly sexual, the 
sexes are always in different individuals, and in no case are 
compound organisms produced by a process of budding or fis- 
sion. Most are ovifarous, producing eggs from which the young 
are developed. Some retain the eggs within the body till the 
young are ready to be hatched, and these are sometimes said to 
be ovo-vivifarous. ‘The higher Vertebrates, however, bring 
forth their young alive, and are said to be viviparous (Latin, 
vevus, living ; and fare, | bring forth). 
PRIMARY DIVISIONS OF THE VERTEBRATA.—The Vertebrata 
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