VERTEBRATA. 



the young animal is excluded from the egg within the 

 body of the parent, while in others the egg itself is 

 produced, and the young hatched some time after- 

 wards. 



This great division embraces four subdivisions or 

 classes, characterised by the kind or power of their 

 motions, which depend on the quantity of their re- 

 spiration. 



I. MAMMALIA. In these the circulation is double, 

 all the blood passing through the respiratory organ, 

 previously to resuming its former course through the 

 arteries : the respiration is aerial and simple ; that is, 

 performed in the lungs only. 



II. BIRDS. Circulation double ; respiration aerial, 

 performed in many other cavities besides the lungs ; 

 the air penetrating their bodies, and bathing the great 

 arteries. 



III. REPTILES. Circulation simple, part only of 

 the blood passing through the respiratory organ, the 

 remainder returning directly to the body. 



IV. FISHES. Circulation double ; but respiration 

 aquatic, and therefore small in quantity. 



From the respective amount of respiration in each 

 class, result the different sorts of motion for which 

 they are chiefly designed : Mammals, in which the 

 quantity is moderate, have (generally) the precision 

 and vigour requisite for walking and running : Birds, 

 which have a greater quantity, possess muscular 

 strength and lightness sufficient for flight : Reptiles, 

 in which it is diminished, usually creep, and many 

 pass a portion of life in a state of torpor : while 



