MAMMALS constitute the highest class of vertebrates and may be defined as warm-blooded creatures in which the outer skin never takes the form of feathers, and in which the young are nourished for a longer or shorter period after birth on their mother’s milk. Hair is rightly considered a very characteristic feature of mammals, though it may take several forms such as the spines of a porcupine or hedgehog, or the scales of some of the ant-eaters. In the case of the armadillo a bony-plated armour has been developed. This is the only class of mammal which has adopted this feature.
With a few exceptions all mammals have the same number of joints in the neck—that is, seven, and this number is not affected by the length of the neck. The normal number of limbs is four, hence the common term ‘quadruped.’ In the case of the whales the hind limbs are mere rudiments.
The first and lowest order of mammals has only two types, the duck-billed platypus and the echidna, or spiny ant-eater. The first named is a most extraordinary creature; its body is like that of a mole covered with short brown fur, the feet are webbed, and it has a short, flat tail. The jaws are extended to form a beak like that of a duck. It burrows into the banks of streams and lives chiefly upon insects.
The echidna is rather like a large hedgehog with a snoutlike bill, no teeth, and a long, sticky tongue with which it catches insects. Its strong claws enable it to dig very easily. It lays eggs, but suckles its young in true mammalian fashion.