The furnishings of the mouth of the whales include sundry remarkable structures peculiar to a certain family circle of these animals. Such are the "whalebone"-plates, furnishing a substance familiarly spoken of by everybody, but exemplifying at the same time a kind of material regarding the origin of which a tacit ignorance, sanctioned by the stolid indifference of many years' standing, commonly prevails. Whalebone, or "baleen," is a commodity occurring in one group of these animals only, this group being that of the whalebone whales (Balænidæ), of which the Greenland or Right whale (Balæna mysticetus) is the most noteworthy example. From this whale the whalebone of commerce is derived; other and nearly related species—such as the Rorquals and Furrowed whales—possessing the whalebone-plates in a comparatively rudimentary state. The baleen occurs in the mouth of these whales, and is disposed in a curious fashion. It exists in the form of flat plates of triangular shape, each plate being fixed by its base in the palate. The inner side, or that next the center of the mouth, is strongly fringed by frayed-out whalebone fibers, the outer edge of each plate being straight. A double row of these triangular plates of baleen depends in the form of two great fringes from the palate of the whale; and it would appear that each baleen-plate is in reality a compound structure, being composed of several smaller plates closely united. The largest plates lie to the outer side of the series, and in a full-grown whale may measure from eight to fourteen feet in length, and as many as 250 or 300 plates may exist on each side of the palate.
The nature of these curious organs forms an appropriate subject of inquiry. It is exceedingly rare in nature to find an animal provided with organs or structures which have no affinity with organs in other and related animals. On the contrary, the principle of likeness or "homology" teaches us that the most unwonted and curious structures in animal existence are for the most part modifications of common organs, or at any rate of parts which are represented under varying forms and guises in other animals. By aid of such a principle we discover that the fore-limb of a horse, the wing of a bird, and the paddle of a whale are essentially similar in fundamental structure, and in turn agree in all necessary details with the arm of man. Through the deductions of this science of tracing likenesses and correspondences between the organs of different animals, the zoölogist has been taught that the "air-bladder" or "sound" of the fish is the forerunner of the lung of higher animals–an inference proved by the fact that in some fishes, such as the curious Lepidosirens or "mud-fishes" of Africa and South America, the air-bladder actually becomes lung-like, not merely in form but in function also. By means of this useful guide to the mysteries of animal structure, we note that the bony box in which the body of the tortoise or turtle is contained is formed by no new elements or parts, but consists chiefly of the greatly modified backbone and of the ribs
