CLASSES OF THE VERTEBRATE!) ANIMALS. 69 



is usually collected in a reservoir named the 

 bladder. 



The sexes in this division are separated. The 

 female has always one or two ovaries, from which 

 the eggs are detached at the moment of conception. 

 The modes in which fecundation is performed are 

 various in the different classes. 



Subdivision of the vertebrated Animals into four Classes. 



We have just considered the general resemblance 

 existing throughout the vertebrated division of the 

 animal kingdom. The various beings of which this 

 division is composed present, nevertheless, to our 

 notice, certain differences or peculiarities which 

 give rise to four principal divisions or classes. 

 These differences depend upon the nature r arid the 

 energy of their movements, which again are, always 

 proportioned to the quantum of respiration, for it is 

 respiration which imparts to the muscular fibre t\{e 

 degree of its irritability through the mediumofthe 

 blood and nervous system. 



The quantity of respiration depends upon two 

 causes: first, the relative portion of blood contained 

 at every given instant of time in the respiratory 

 organ ; and secondly, the amount of oxygen which 

 enters into the composition of the circumambient 

 fluid. 



The quantity of blood is altogether determined 

 by the peculiar disposition of the organs of respira- 

 tion and of circulation. 



