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CHAPTEE XIII. 



CLASSIFICATION OF THE CKANIOTA. 



A. THE ICHTHYOPSIDA. 



Craniote animals are divided into five great classes — the Pisces, 

 Amphibia, Reptilia, Aves, and Mammalia. There are two im- 

 portant ways of classifying these five sections. One was formu- 

 lated by the late Professor Owen, who divided them into two 

 primary sections, the H(Bmatocrya and the Haematotherma, the 

 distinctive features being taken from the blood. The Hcemafo- 

 crya, or the Fish, Amphibia, and EeptUia, have an imperfect 

 circulatory system, there never being four distinct chambers to 

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

 Ho&maiotherma 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 imnatural 

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

 and Eeptilia, 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. 



By far the most rational classification is that given by Hux- 

 ley, who divided the Craniota into three sections, as follows : — 



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

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

 corpuscles are always, as far as we can see, nucleated. The em- 

 bryo has never that foetal membrane called the amnion, and the 

 allantois when present is only rudimentary. Diaphragm absent. 



