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CHAPTER XIII. 



CLASSIFICATION OF THE CEANIOTA. 



A. THE lOHTHYOPSIDA. 



The Craniota are divided into five great classes — the Pisces, 

 Amphibia, Reptilia, Aves, and Mammalia. Tliere are two 

 ways of classifying these five sections. One was formulated 

 by the late Professor Owen, who divided them into two 

 primary sections, the Hcumatocrya and the Hmmatotha'ma, the 

 distinctive features being talten from the blood. The Haimato- 

 cnja, or the Fish, Amphibia, and Peptilia, have an imperfect 

 circulatory system, there never being four distinct chambers to 

 the heart. These are called Cold - blooded Animals. The 

 Hwinatotherma are the Aves and Mammalia, in which we 

 always find four distinct chambers to the heart and a complete 

 pulmonary and systemic circulation. Circulation is rapid. These 

 are the Warm-blooded Animals. This division is an unnatural 

 one ; for we find placed in the two separate groups the Birds 

 and Reptilia, which to a certain extent interlace when we ex- 

 amine the fossil species in the rocks, and which even in existing 

 forms present some analogous structural features. 



Huxley divided the Craniota into three sections, as 

 follows : — 



A. Ichthyopsida = Fish and Amphibia. — These have always 

 gills or branchiae at some period of their life, and their blood- 

 corpuscles are nucleated. The embryo has never that foetal 

 membrane called the amnion, and the allantois when present 

 is only rudimentary, Uiajjhragm absent. 



