The sub-kingdom Vertebrata is divided into the five great classes of the Fishes (Pisces), Amphibians (Amphibia), Reptiles (Reptilia), Birds (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 Vertebrata is to divide them into the two primary sections of the Branchiata and Abranchiata. Of these, the Branchiate section 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 characterised 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 allantois. 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 Haematocrya and the Haematotherma, the characters of the blood being taken as the distinctive character. The Haematocrya or Cold-blooded Vertebrates comprise the Fishes, Amphibia, and Reptiles, and are characterised by their cold blood and imperfect circulation. The Haematotherma 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 pulmonary and systemic circulations. The chief objection to this division lies in the separation which is effected between the Reptiles and the Birds, two classes which are certainly very nearly allied to one another.

* 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 is 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 increases 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.

By Professor Huxley the Vertebraia are divided into the following three primary sections: