INTRODUCTION.
The Animal Kingdom was divided by Cuvier into four great divisions or sub-kingdoms—Vertebrata, Mollusca, Articulata, and Radiata. The last of these has been greatly divided of late, and the limits of two of the others have been slightly altered. The Vertebrata are essentially distinguished from the others by the possession of an internal osseous frame or skeleton, enclosing a distinct brain and spinal cord. They never have more than four limbs; the mouth consists of two jaws placed vertically, the blood is red, and they have distinct organs of vision, hearing, taste, and smell.
Vertebrate animals comprise four distinct classes, some of which are cold-blooded, i.e., with blood nearly the temperature of the surrounding medium,—Fishes and Reptiles; whilst the others are warm-blooded,—Birds and Mammals. The former have been already treated of in this series.
Mammals are warm-blooded, viviparous animals, and are distinguished from Birds, as well as from the other vertebrated animals, by the possession of mammary glands, secreting a nutritious fluid called milk, for the nourishment of their young, and terminating outwardly in all (except one or two) by teats. They are also distinguished by a covering of hair, entire or partial. Whales appear to be exceptional; but even in these the fœtus has some tufts of hair.
The lungs are free, and are separated from the abdominal viscera by a muscular partition called the Diaphragm, which is also one of the chief agents for inspiration. Most animals have four limbs; hence the old term Quadruped, which, on account of its non-applicability to the Cetacea, has properly been allowed to become obsolete.
Mammals, says Cuvier, require to be placed first, because they
