Order CETACEA.
Whales, dolphins, and porpoises coustitute an order diifering widely, both in form and structure, from all land-mammals. They were at one time supposed to have some affinity with the Carnivora, but Flower has shown that the relationship is doubtful, oud that Cetaceans are probably more nearly allied to some of the primitive Ungulates than to any other Mammalia.
The Cetacea are modified for a purely aquatic life and their external form much resembles that of Fishes. There are no external hind limbs, whilst the tail is flattened and expanded into lobes, known as flukes, so as to resemble that of a fish in outline, though the expansion is horizontal instead of vertical. The anterior limbs are converted into paddles, termed flippers or pectoral fins, the digits being completely united together by skin and destitute of nails. There is in most genera a dorsal fin composed of integument. The skin is smooth and hairless, With the exception of a few bristles round the mouth, generally confined to young animals; but the body is surrounded, immediately beneath the skin, by a thick layer of fat or "blubber," which, like the hair or wool of land-animals, serves to retain the heat of the body.
The eye is small and the ear-orifice minute; there is no trace of an external ear. The nostrils open either separately or by a single, generally crescentic, orifice or "blow-hole" much above the extremity of the snout, and in most forms on the top of the head. The mammæ, two in number, discharge each by a teat lying in a groove, one on each side of the genital orifice. The testes are abdominal, the uterus is bicornuate, the placenta non-deciduate and diffuse.
The peculiarities of the skeleton are too numerous for any except the most important to be here mentioned. The bones generally are spongy in texture, the cavities being filled with oil. The skull is greatly modified and consists of a short, almost round brain-case, and of a more or less elongate rostrum. The cervical vertebrae are often partially or wholly anchylosed. There is no sacrum. The mode of attachment of the ribs to the vertebræ is more or less peculiar, and presents modifications characteristic of the different families. There are no clavicles. The radius and ulna are distinct, and are flattened, as are all the bones of the wrist and hand. The digits are 4 or 5 in number, more often the latter, and the phalanges of the second and third digits greatly exceed in number those found in other mammals. A pair of styliform bones represent the pelvis.
