But the arm, which in man moves freely, is here chained to the body as far as the hand, and the latter, which, in obedience to human volition and intellect, executes such miracles of industry and art, is here covered with a thick skin, and appears as a broad undivided fin or flapper. Yet still it is destined for higher service than that of a mere propelling oar, as it serves the mother to guide and shield her young. The lower extremities are of course wanting, but their functions are performed by the mighty _horizontal_ tail, by whose powerful strokes the unwieldy animal glides rapidly through the waters. The cetaceans distinguish themselves, moreover, from the fishes by the bringing forth of living young, by a greater quantity of blood, by the smoothness of their skin, under which is found a thick layer of fat, and by their simple or double blow-hole, which is situated at the top of the head, and corresponds to the nostrils of the quadrupeds, though not for the purpose of smelling, but merely as an organ of respiration. [Illustration: Bones of the Anterior Fin of a Whale.] Our knowledge of the cetaceans is still very incomplete; and this is not to be wondered at, when we consider that they chiefly dwell in the most inaccessible parts of the ocean, and that when met with, the swiftness of their movements rarely allows more than a flighty view of their external form. Thus their habits and mode of living are mostly enveloped in obscurity; and while doubtless many cetaceans are to the present day unknown, one and the same species has not seldom been described under different names, to the no small confusion of the naturalist. The cetaceans are either without a dental apparatus, or provided with teeth. The former, or the whalebone whales, have two blow-holes on the top of the head, in the form of two longitudinal fissures; while in the latter, (sperm-whales, unicorn-fish, dolphins,) which comprise by far the greater number of species, there is but one transversal spout-hole. In all whales the larynx is continued to the spouting canal, and deeply inserted or closely imbricated within its tube. Thus no tones approaching to a voice can be emitted except through the spiracles, which are encumbered with valves, and evidently badly adapted for the transmission of sound. Scoresby assures us that the Greenland whale has no voice, and Bennett frequently noticed sperm-whales suffering from extreme alarm and injury, but never heard any sound from them beyond that attending an ordinary respiration.