458 MANUAL OF ZOOLOGY. 



(Aves), and Mammals (Mammalia). So far there is perfect 

 unanimity ; but when it is inquired into what larger sections 

 the Vertebrata may be divided there is much difference of 

 opinion. Here, the divisions proposed by Professor Huxley 

 will be adopted ; but it is necessary that those employed by 

 other writers should be mentioned and explained. 



One of the commonest methods of classifying the Verte- 

 brata is to divide them into the two primary sections of the 

 Branchiata and Abranchiata. Of these, the Branchiate sec- 

 tion includes the Fishes and Amphibians, and is characterised 

 by the fact that the animal is always provided at some period 

 of its life with branchiae or gills. The Abranchiate section 

 includes the Reptiles, Birds, and Mammals, and is character- 

 ised by the fact that the animal is never provided at any time 

 of its life with gills. Additional characters of the Branchiate 

 Vertebrates are, that the embryo is not furnished with the 

 structures known as the amnion and allanto'is. Hence the 

 Branchiate Vertebrates are often spoken of as the Anamniota 

 and as the Anallantoidea. In the Abranchiate Vertebrates, 

 on the other hand, the embryo is always provided with an 

 amnion and allantois, and hence this section is spoken of as 

 the Amniota or as the Allantoidea.* 



By Professor Owen the Vertebrata are divided into the two 

 primary sections of the H&matocrya and the Hcematotherma, 

 the characters of the blood being taken as the distinctive 

 character. The Hcsmatocrya or Cold-blooded Vertebrates 

 comprise the Fishes, Amphibia, and Reptiles, and are charac- 

 terised by their cold blood and imperfect circulation. The 

 Hcematotherma or Warm-blooded Vertebrates comprise the 

 Birds and the Mammals, and are characterised by their hot 

 blood, four-chambered heart, and complete separation of the 

 s j:>ulmonary and systemic circulations. The chief objection to 

 this division lies in the separation which is effected between 



* The amnion (fig. 239, C) is a membranous sac, containing a fluid the 

 liquor amnii and completely enveloping the embryo. It constitutes 

 one of the so-called "foetal membranes," and h thrown off at birth. The 

 allantois (fig. 239, C) is an embryonic structure, which is developed out of 

 the middle or " vascular" layer of the germinal membrane. It appears at 

 first as a solid, pear-shaped, cellular mass, arising from the under part of 

 the body of the embryo. In the process of development, the allantois in- 

 creases largely in size, and becomes converted into a vesicle which envelops 

 the embryo in part or wholly. It is abundantly supplied with blood, and 

 is the organ whereby the blood of the foetus is aerated. The part of the 

 allantois which is external to the body of the embryo is cast off at birth ; 

 but the portion which is within the body is retained, and is converted into 

 the urinary bladder. 



