VERTEBRATE ANIMALS. 



3; 



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 sjjine 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 VEETEBRATE 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 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 quantit}^ of blood which is thrown 

 into the lungs at each pulsation. 



f^ishes 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, sujicrior 

 to that of rejitiles, 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 



atrachian reptiles (frogs, 

 xteiit performed over the 



action without previous 



vliole outer «kiu ; whii;U, ( 

 St. Hence, as there can 

 cspiratiuii) the chemical t 



effected hj' which is neeiled to developc the requisite nervoiis or vitiU 

 enerjfy, those animals of this group which in the ailult state have 

 lungs and not gills, but M'hich pass the winter in a torpid state under 

 water, art. t^naUled to resuscitate in spring.^Ew. 



