In this order are the Whales, Dolphins, and Porpoises, all agreeing with the preceding in their complete adaptation to an aquatic life (figs. 384, 386). The body is completely fish-like in form; the anterior limbs are converted into swimming-paddles or "flippers;" the proximal bones of the fore-limbs are much reduced in length, and the succeeding bones are shortened and flattened, and are enveloped in a tendinous skin, thus reducing the limbs to oar-like fins, the phalanges of some of the digits being sometimes increased in number (fig. 381); there are no external ears ; the posterior limbs are completely absent; and there is a powerful, horizontally -flattened, caudal fin, sometimes accompanied by a dorsal fin as well. In all these characters the Cetacea agree with the Sirenia, except in the one last mentioned. On the other hand, the nostrils, which may be single or double, are always placed at the top of the head, constituting the so-called "blow-holes" or "spiracles;" and they are never situated at the end of a snout. The body of the adult is in general completely hairless. The testes are retained throughout life within the abdomen, and there are no vesiculae seminales. The teats are two in number, and are placed upon the groin. The head is generally of disproportionately large size, and is never separated from the body by any distinct constriction or neck. The lumbar region of the spine is long, and, as in the Sirenia, there is no sacrum, and the pelvis is represented by a single bone (the ischium) on each side. A rudimentary femur maybe present, and Balaena mysticetus has a cartilaginous tibia as well. There are no clavicles, and the sternum is broad and flat in form. Lastly, the adult is either destitute of teeth, or, with the single exception of the Zeuglodontidae, is monophyodont - that is to say, possesses but a single set of teeth, which are never replaced by others. When teeth are present, they are usually conical and numerous, and, except in the Zeuglodonts, they are always of one kind only.
The skull is often unsymmetrically developed, and the maxillae and praemaxillae are greatly prolonged. The nasal bones are short, and the nasal passages are vertically directed; the epiglottis and laryngeal cartilages being prolonged behind the soft palate in the form of a cylindrical tube, which is practically continuous with the posterior nares, thus allowing the animal to swallow under water without choking.
The Cetacea may be divided into the five families of the Balae-nidae or Whalebone Whales, the Delphinidae or Dolphins and Porpoises, the Catodontidae or Sperm Whales, the Rhynchoceti or Ziphioid Whales, and the Zeuglodontidae. Of these the Balae-nidae are often spoken of as the " toothless" Whales, whilst the other four families are called the "toothed" Whales (Odonloceti).
Fam. 1. Balaenidae. - The Balaenidae or Toothless Whales are characterised by the total absence of teeth in the adult (fig. 382). Teeth are, however, present in the foetal Whale, but they never cut the gum. The place of teeth is supplied by a number of plates of whalebone or "baleen" attached to the palate; hence the name of "whalebone whales" often given to this family. They are the largest of living animals, and may be divided into the two sections of the Smooth Whales, in which the skin is smooth and there is no dorsal fin (as in the Greenland Whale), and the Furrowed Whales, in which the skin is furrowed and a dorsal fin is present (as in the so-called Finner Whales and Hump-backed Whales).
Fig. 382. - Skull of the Right Whale (Balaena mysticetus). (After Owen.)
The Greenland or ' Right" Whale (Balaena mysticetus) will illustrate almost all the leading points of interest in the family. The Greenland Whale is the animal which is sought after in the whale-fishery of Europe, and hence the name of "Right" Whale often applied to it. It is an inhabitant of the arctic seas, and reaches a length of from forty to sixty feet. Of this enormous length, nearly one-third is made up of the head, so that the eye looks as if it were placed nearly in the middle of the body. The skin is completely smooth, and is destitute of hairs in the adult. The fore-limbs are converted into "flippers" or swimming-paddles, but the main organ of progression is the tail, which often measures from twenty to twenty-five feet in breadth. The mouth is of enormous size, the upper jaw much narrower than the lower, and both completely destitute of teeth. Along the middle of the palate runs a strong keel, bordered by two lateral depressions, one on each side. Arranged transversely in these lateral depressions are an enormous number of horny plates, constituting what is known as the "baleen" plates, from which the whalebone of commerce is derived. The arrangement of the plates of baleen is as follows (fig. 383): Each plate is triangular in shape, the shortest side or base being deeply sunk in the palate. The outer edge of the plate is nearly straight, and is quite unbroken. The inner edge is slightly concave, and is furnished with a close fringe formed of detached fibres of whalebone. For simplicity's sake each baleen-plate has been regarded here as a single plate, but in reality each plate is composed of several pieces, of which the outermost is by far the largest, whilst the others gradually decrease in size towards the middle line of the palate. The large marginal plates are from eight to ten or more feet in length, and there may be over one hundred on each side of the mouth.