CETA'CEA (Gr. kelos, a whale), an order of mammalia (q.v.) greatly differing in gen eral form and habits from the rest of that class, so as indeed to be popularly reckoned among fishes. The C. have a fish-like form, terminating in a fish-like tail or tail-fin, which, however, is not vertical, as in fishes, but horizontal, and is the great instrument of progression; being moved by very powerful muscles, commonly with an oblique downward and lateral movement, like that by which a boat is propelled in sculling, but sometimes by direct upward and downward strokes, when greater velocity is requisite. There are no hinder limbs, and even the pelvis is represented only by two small rudi mentary bones, suspended in the soft parts, so that the body tapers gradually and unin terruptedly towards the tail. The fore-limbs arc exclusively, or almost exclusively, adapted for swimming; their bones, however, appearing in the skeleton as those of a hand, placed at the extremity of an arm, of which the bones are much abbreviated and consolidated, with little power of motion except at the shoulder-joint, and are entirely concealed in the soft parts of the animal. The head is connected with the body with out any apparent neck, and the vertebrae of the neck are partly ankylosed or soldered together. The skin is naked, having no general covering of hair, although sonic of the species possess conspicuous whiskers. The C. agree with quadrupeds, notwithstanding the great differences already indicated, in the most important parts of their organiza tion. They are viviparous, and suckle their young, for which they exhibit went affec tion; they are also warm-blooded, breathe by lungs, and not by gills, and come to the surface of the water for the purpose of inhaling air. An approach to their fish-like form is to be seen in seals (q. v.) and other phoeides (q.v.); in which, however, the hinder limbs are largely, although peculiarly developed, whilst the fish-like tail-fin is wanting; the skin has a covering of hair; and the head and fore-limbs more resemble those of ordinary quadrupeds.
The C. are usually divided into two sections—the herbivorous and the ordinary C.; but the former, constituting the family of manatidce (q.v.), have recently, by sonic systematic naturalists, been rejected from this order altogether, and associated with the pachydermata. They differ very widely from the ordinary or true C., not only in their adaptation for the use of vegetable instead of animal food, which appears both in their dentition and in their digestive apparatus, but also in their pectoral instead of abdomi nal teats, and in their want of blow-holes and of any provision for retiring to great depths of the ocean, and remaining there for a considerable time, without returning to the surface to breathe.
The ordinary or tree C. are divided into the families of delphinida (dolphin, por poise, beluga, bottlenose, narwhal, etc.), physeterido3 or catodontidm (cacholot, or sperma ceti, whale, etc.), and bakenides (Greenland whale, rorqual, etc.), the distinguishing characters of which are given under separate heads. They all feed on animal food, some of them pursuing and devouring fishes; others, and these the largest, subsisting chiefly on smaller prey, mollusks, small crustaceans, and even zoophytes, which they strain out of the water by a peculiar apparatus in their mouths. None of the true C. have molar teeth or grinders like the manatidee; all the teeth which any of them have are conical; but some of the largest are entirely destitute of teeth. The females of all of them have the teats situated far back on the abdomen. The fore-limbs of the true C. are mere fins, the slight power of grasping with them, which the Inanatidw pos sess, having entirely disappeared. The resemblance to fishes is increased in many of them by the presence of a dorsal fin. There is a wonderful provision to enable them to spend some time under water, before returning again to the surface to breathe—an arterial plexus or prodigious intertwining of branches of arteries, under the pleura and between the ribs, on each side of the spine. This being filled with oxygenated blood, after the animal has spent some time at the surface breathing, the wants of the system are supplied from it, whilst breathing is suspended, so that some whales can remain below even for an hour. The position of the nostrils is remarkable, almost on the very top of the head, so that the animal can breathe as soon as the head comes to the surface of the water; and the nostrils are furnished with a valve of singular but very perfect con struction, a sort of conical stopper of fibrous substance, preventing the ingress of water even under the pressure of the greatest depths. The nostrils appear to be little used for the purpose of smelling, the sense of smell being one which these animals either do not possess at all, or in a very imperfect degree; but they are much used, not only for breathing, but also for spouting, or the ejection of water from the mouth, for which reason they are generally called blow-holes—the water being forced through them by the compression of two large pouches or reservoirs which are situated beneath them. This. compression is accomplished by an action similar to that of swallowing; the throat, however, not being open, but closed. The height to which the water is thrown into the air is extraordinary, and the spouting of the whale is one of those wonders of the ocean never to be forgotten by those who have seen it.