VERTEBRATE ANIMALS. 



37 



All these animals have a particular secretion, which is that of urine, and which is 

 elahorated 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 threi 

 first classes, it requires an intromission of the fluid ; in some reptiles, and in most ol 

 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 i 

 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 which 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 respirator)' 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 ;is 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 Betnchlen reptiles (froffs, newts, tic), respiration is to a 

 certain extent performed over the whole outer sLm , which, on thle 



aeeount, requires to be always mulat. llen.e, as there run lie no 

 niiiM uUr aet without previous respiration, the chemical change 



effected by which i* n led to drrmlopc the i vu of *itaJ 



energy, thoee enlneli ol tin. group which In the adult state hare 

 lunge end not gilli, hut which peel the ■ Inlet In • tor| id state under 

 water, are enabled to rcsuscita' 



