Cetacea, an order of mammals, of aquatic habit and fish-like form. The head is large, the neck indistinct; there is generally a median dorsal fin, and the tail has lateral flukes; the fore-limbs are reduced to paddles, the hind-limbs are at most represented by slight internal traces; the skin is smooth, and, with the occasional exception of a few bristles near the mouth, hairless; there is a thick layer of fat or blubber under the skin which serves instead of hair as a heat-retainer. The eye is small, there is no external ear, the nostrils are situated vertically. The bones are spongy and oily, the neck vertebrae are compressed and often fused, there is no union to form a sacrum. The skull is peculiarly modified, the brain-case being high, and the front part prolonged into more or less of a snout. There are no collar-bones; the bones of the arm are flattened and stiff; the joints of the second and third fingers are always above the normal number; the whole arm forms a flipper; the hip-girdle and hind-leg are degenerate. In one group teeth are absent except in the foetns, and are replaced by 'whalebone' growths from the palate; in no case is there more than one set of teeth. The stomach has several chambers; the intestine is simple. The liver is less divided than usual, and there is no gall-bladder. The blood-vessels form wonderful networks (retia mirabilia). The top of the windpipe is prolonged forwards so as to form, when embraced by the soft palate, a continuous air-passage from nostrils to lungs. The brain is large. The placenta is 'non-deciduate and diffuse.' The teats are two in number, and lie beside the female genital aperture; the milk is squeezed into the mouth of the sucking young.
The Cetacea are widely distributed in all seas and in some large rivers. They swim powerfully, and the tail works up and down, not sideways. They rise to the surface to breathe, and do not spout sea-water from their blowholes. The expiration is periodic and violent, and the forcibly expelled air being laden with water, vapour may condense in a pillar of fine spray, or the ascending column may carry up some surface sea-water along with it, but it must be recognised that the process is simply that of ordinary expiration in peculiar conditions. They are mostly inoffensive, generally social in habit, vary from 4 to 60 feet in length, and feed on jelly-fish, crustaceans, pteropods, cuttlefish, fishes, and in one genus (Orea) on seals and on other whales.
Two very distinct series have to be distinguished —(a) the Toothed Whales or Odontoceti, and (b) the Baleen Whales or Mystacoceti. The former include Sperm Whales (Physeter), the Bottlenose (Hyperoodon), the genus Platanista and its allies, and the great family of Dolphins (q.v.). The latter sub-order includes the Right Whale (Balaena), the 'Humpbacks' (Megaptera), and the Rorquals (Balaenoptera).
In the Eocene, Cetacea are represented by primitive, less specialised forms, known as Zeuglodons, but the remains are, as one would expect, somewhat fragmentary, and the conclusions to be drawn from them very uncertain. In Miocene and Pliocene strata still more fragmentary cetacean remains have been found, and are grouped together in the genus Squalodon.
There is much doubt and dispute in regard to the origin and affinities of Cetacea. They are related by some to Carnivores, but the researches of Professor Flower have made it more probable that they have much closer affinities with Ungulates. He regards it as not unlikely that the whole group had a fresh-water origin. Fuller details must be sought under the article WHALE. See Flower's article 'Mammalia,' Ency. Brit.