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ANIMALIA VERTEBRATA. 29 
which, after being united in a trunk called the vena-porte, is again sub- 
divided at the liver. 
All these animals have a particular secretion; the wrine, which is pro- 
duced in two large glands, attached to the sides of the spine of the back, 
called kidneys—the liquid they secrete is most commonly poured into a 
reservoir, named bladder. 
The sexes are separate, and the female has always one or two ovaries, 
from which the eggs are detached at the instant of conception. The male 
fectsadifies them with the seminal fluid, but the mode varies greatly. In 
most of the genera of the three first classes, it requires an intromission of 
the fluid; in some reptiles, and in most of the fishes, it takes place after 
the exit of the egg. 
Subdivision of the Vertebrata into four Classes. 
We have just seen how far vertebrated animals resemble each other, 
they present, however, four great subdivisions or classes, characterised by 
the kind or power of their motions, which depend themselves on the quan- 
tity of their respiration, inasmuch as it is from this respiration that the 
muscular fibres derive the strength of their irritability. 
The quantity of respiration depends upon two agents: the first is the 
relative amount of blood which is poured into the respiratory organ in a 
given instant of time; the second is the relative amount of oxygen which 
enters into the composition of the surrounding fluid. The quantity of the 
former depends upon the disposition of the organs of circulation and re- 
spiration. 
The organs of the circulation may be double, so that all the blood 
which is brought back from the various parts of the body by the veins, is 
forced to circulate through the respiratory organ, previous to resuming 
its former course through the arteries; or they may be simple, so that a 
part only of the blood is obliged to pass through that organ, the remainder 
returning directly to the body. 
The latter is the case with reptiles. The quantity of their respiration, 
and all their qualities which depend on it, vary with the amount of blood 
thrown into the lungs at each pulsation. 
Fishes have a double circulation, but their organ of respiration is formed 
to execute its function through the medium of water; and their blood is 
only acted on by the portion of oxygen it contains, so that the quantity of 
their respiration is perhaps less than that of reptiles. 
In the mammalia the circulation is double, and the aerial respiration 
simple, that is, it is performed in the lungs only; their quantity of respi- 
ration is, consequently, superior to that of reptiles, on account of the form 
