CETACEA, se-ta'se-ii, an order of mam mals whose structure is so modified as to render them fit for an aquatic life. The whalebone whales, the toothed whales, as the porpoise, narwhal, etc., and the extinct zeuglodon, rep resent the leading divisions of the group. The body is fish-like in form, the head passing grad ually into the trunk which tapers posteriorly and ends in a bilobate caudal fin which is placed horizontally, not as in the fishes, vertically. The posterior limbs are wanting, and the anterior are converted into broad paddles or flippers, consisting of a continuous sheath of the thick integument, within which are present repre sentatives of all the bones usually found in the fore limb of mammals, but they are not mov ably articulated, so that the paddle moves like a solid oar. The fish-like aspect is further in creased by the presence of a dorsal fin; but this is a simple fold of integument, and does not contain, as in fishes, any bony spines. The vertebra of the neck, seven in number, are united more or less to each other, so that in some they form a single solid piece. The right whale and its allies have no teeth in the adult state, their place being taken by the triangular plates of baleen or whalebone which are de veloped on transverse ridges of the palate. The frayed edges of these plates slope obliquely downward and outward from the middle of the roof of the mouth, so that when the mouth is shut there is a triangular space in the middle, the floor of which is formed by the enormous tongue. The water taken into the mouth is sifted by the frayed edges of the plates; it is driven out sideways between the plates and the tongue sweeps backward to the gullet any animals that have been caught in the fringes. But the fetal whales possess minute teeth, which are very soon lost. The porpoises, etc., when they possess teeth in one or both jaws, have them numerous and conical in form; they have no milk predecessors. The stomach is divided into several chambers, but these are not, as in rumi nants, connected direCtly with the gullet; they are rather appendages of the pyloric portion of the organ.
The arrangement of the respiratory and cir culatory systems, which enable the Cetacea to remain for some time under water, are inter esting. The nostrils open directly upward on
the top of the head, and are closed by valvular folds of integument which are under the control of the animal. When the animal comes to the surface to breathe it expels the air violently, and the vapor it contains becomes condensed into a cloud; if the expiration commences be fore the mouth of the spiracle or blow-hole is above the surface, a little water may be brown up like spray but no water from the mouth is thus discharged, for the soft palate firmly em braces during life the upper end of the larynx, so that the gullet is divided into two narrow passages, while the lungs have a continuous passage to the exterior. The blood vessels, especially those of the thorax and spinal canal, break up into extensive plexuses or networks, in which a large amount of oxygenated blood is delayed, and thus the animal is enabled to remain under water, the necessity for changing the air in the lungs being diminished.
Fossil Bones of cetaceans, mostly allied to the living species, are found in the marine sediments of the Tertiary and Quater nary ages, and are occasionally dredged up from deep-sea deposits. The hard and heavy ear bones are especially apt to be preserved as fos sils. The zeuglodons and squalodons of the Eocene epoch represent a peculiar primitive group of cetaceans with two-rooted teeth. Very little is known of the evolution of this order of mammals. The Tertiary .deposits of Pata gonia are supposed to be rich in material bear ing on the evolution of the Cetacea.
Classification of The of cetacea are as follows : 1. Mystacoceti, baleen-bearing whales. Its families are: Balcenopteridee, rorquals, and other great whalebone whales; Balcenids, right whales and kogias.
2. Odontoceti, of which the families are: Physetericke, sperm-whales (Physeterince), and beaked whales (Ziphiince); Delphinidee, dol phins, porpoises, white whales, killers and the like; Platanistide, river dolphins; and the ex tinct Sgualodontidce.
3. Archsoceti, containing the extinct family Zeuglodontidce. The most recent and import ant work on this order is Beddard's (Book of Whales' (London 1900). See WHALE, and the names of the various groups and species of cetaceans.