DIVISION OF VERTEBRATA INTO CLASSES. 



91 



the body, not being in any instance diffused or scattered through- 

 out the whole, as in Insects and some other tribes. In all but 

 the lowest group, namely, Fishes, and the Batrachians (Frogs, 

 Newts, &c.) during their larva state, the organs of respiration 

 are adapted to breathe air ; but in those, the aquatic respiration, 

 characteristic of the Molluscous classes, is still retained. In the 

 classes in which the respiration is carried on most actively, and 

 in which the whole mass of the blood is exposed to its influence, 

 before again circulating through the system, namely, Mammals 

 and Birds, the animal has the power of maintaining an elevated 

 temperature, independent of that of the surrounding air; and 

 they are termed warm-blooded from this circumstance ; those in 

 which the temperature of the body varies with that of the sur- 

 rounding air or water, being cold-blooded. There is a further 

 essential distinction among the different classes of Vertebrata, 

 dependent on the mode in which the function of reproduction is 

 performed in them ; this takes place by eggs in the four lowest 

 classes; but in the highest, that of Mammals, the young are 

 born alive, and are nourished afterwards by suckling. 



70. It is upon the mode in which the functions of Circula- 

 tion, Respiration, and Reproduction, are performed, in the differ- 

 ent groups of Vertebrata, that their division into classes is 

 founded ; and the following table will show the principles on 

 which this is accomplished ; besides furnishing some other char- 

 acters, which have not been here adverted to, but which will be 

 hereafter considered in detail, under their respective heads : 



Oval in a few exceptions. 



