VERTEBRATE ANIMALS. 
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All these animals have a particular secretion, which is that of urine, and which is 
elaborated in two large glands attached to the sides of the spine of the back, and called 
kidneys : the liquid which these glands secrete, accumulates most commonly in a 
reservoir named the 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 fecundates 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 eggs. 
SUBDIVISION OF THE VERTEBRATE ANIMALS INTO FOUR CLASSES. 
We have seen to what extent vertebrate animals resemble each other : they present, 
however, four great subdivisions or classes, characterized by the kind or power of their 
movements, which depend themselves on the quantity of respiration, inasmuch as it is 
from this respiration that the muscular fibres derive the energy of their irritability. 
The quantity of respiration depends upon two agents : the first is the relative 
quantity of blood which presents itself in the respiratory organ in a given instant of 
time ; the second, the relative amount of [free] oxygen which enters into the com- 
position of [or is dispersed through] the ambient fluid. The quantity of the former 
depends upon the disposition of the organs of respiration and of circulation. 
The organs of the circulation may be double, so that all the blood wdiich is brought 
back from the various parts of the body by the veins, is forced to circulate through 
the respiratory organ before returning by the arteries ; or they may be simple, so that 
a portion only of the blood is obliged to pass through the respiratory organ, the re - 
mainder returning to the body without having been subjected to respiration. 
The latter is the case with reptiles. The amount of their respiration, and all the 
qualities which depend on it, vary according to the quantity of blood which is 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 upon by that 
small portion of oxygen which is dissolved or mingled in water ; so that the quantity of 
their respiration is, perhaps, less than that of reptiles. 
In mammalians, the circulation is double, and the aerial respiration simple, that is, 
it is performed in the lungs only : their quantity of respiration is, therefore, superior 
to that of reptiles, on account of the form of their respiratory organ, and to that of 
fishes, from the nature of their surrounding medium. 
But the quantity of respiration in birds is even superior to that of quadrupeds, 
since they have not only a double circulation and an aerial respiration, but also 
respire by many other cavities besides the lungs, the air penetrating throughout 
their bodies, and bathing the branches of the aorta, or main artery of the body, as 
well as those of the pulmonary artery.* 
Hence result the four kinds of progression to which the four classes of the vertebrate 
animals are more particularly destined. The quadrupeds, in which the quantity of 
* In Batrachian reptiles (frogs, newts, &c.), respiration is to a 
certain extent performed over the whole outer skin ; which, on this 
account, requires to be always moist. Hence, as there can be no 
muscular action without previous respiration, the chemical change 
effected by which is needed to develope the requisite nervous or vital 
energy, those animals of this group which in the adult state have 
lungs and not gills, but which pass the winter in a torpid state under 
water, are enabled to resuscitate in spring. — Ed. 
