First, then, while the young of fishes are almost invariably produced from eggs and are not nourished by the parents, the young of the porpoise are produced alive, and are nourished with milk secreted by the mammary glands of the mother. This is an all-important feature, and is the one implied in the term _mammal_. The porpoise also differs from nearly all fishes in that it breathes by lungs instead of gills, obtaining its air direct from the atmosphere, and not from the water. Hence we find it coming to the surface at frequent intervals to discharge its vitiated air and to inhale a fresh supply. The body-cavity of a mammal is divided into two parts by a muscular diaphragm, the foremost division, called the thorax, containing the heart and lungs, and the other (the abdomen) the remainder of the internal organs, while the diaphragm itself plays an important part in the respiratory movement by which air is drawn into the lungs. The body of the porpoise is so divided, but no such division ever occurs in any of the fishes. Lastly, the heart of the porpoise, in common with the rest of the mammals, is divided into four cavities, and the blood is warm, while the heart of a fish has generally only two divisions, and the blood propelled by it is of about the same temperature as that of the surrounding medium. Several other important differences between the porpoise and the fish might be given, but the above will be quite sufficient to show why they are placed in different classes. Mammals are divided into several classes, and one of these (_Cetacea_) includes the fish-like Whales, Porpoises, and Dolphins, all of which are peculiarly adapted to a purely aquatic life. Like most of the fishes, their upper surfaces are of a dark colour, and the lower very light. Their fore limbs are constructed on the same plan as those of the higher mammals, the bones of the arm being attached to a large shoulder-blade, and the hand formed of four or five well-developed fingers which are enclosed in skin, so that they constitute a paddle or flipper well adapted for propulsion through water. There is no collar-bone, however, and the fingers have no nails or claws. There are no hind limbs visible externally, but a rudimentary pelvic girdle forms a part of the internal skeleton. A dorsal fin exists, but this is merely an extension of the skin of the back, and is not supported by either bones or rays. The skin itself has no scales, like that of most fishes, but is smooth and naked; and below it lies a large amount of fat, which, being a very bad conductor of heat, serves to prevent the escape of heat from the body.