IN 6ENEEAL. 9 



in tlie crocodiles abdominal ribs, which are nothing else than the 

 ossified transverse tendinous strips of the straight muscles of the 

 abdomen. 



The vertebral column and the skull are the only essential parts 

 of the internal skeleton. The limbs are wanting in most snakes 

 and in some fishes, the ribs in the frogs and others. If now from 

 the skeleton of a four-footed mammal or of man the limbs and the 

 ribs be subtracted, then there will remain nothing but the verte- 

 bral column and the bony head. 



The division of vertebrate animals into four classes, derived 

 from LiNN^US, has since his time been generally adopted, and 

 maintained, although some writers have wished to compose a fifth 

 class from the scaleless Amphibia. That division is founded upon 

 the temperature of the blood ^ and of the internal organs, upon the 

 various forms of the heart, upon the difference of the respiratory 

 organs, which are either gills or lungs, and upon the distinction of 

 the parturition, the laying of eggs or the bringing forth of living 

 young. 



These four classes are those of fishes, reptiles, birds and mam- 

 mals. Fishes and reptiles are cold-blooded, birds and mammals 

 warm-blooded. Fishes breathe by gills, the remaining vertebrate 

 animals by lungs ; birds and mammals are thus warm-blooded ani- 

 mals that breathe by lungs ; but of these the first are oviparous, the 

 last viviparous. 



^ If, during life, the temperature of animals undergoes only very little variation, 

 and is almost entirely independent of that of the medium in which they live, then they 

 are said to be warm-blooded. Animals whose temperature is dependent in great part 

 on that of the air or of the water, in which they reside, are, on the other hand, named 

 cold-blooded. 



