CHORD ATA 



Order 3. Cetacea 



373 



The Cetacea (Gr. /o/to?, whale) are aquatic mammals and 

 comprise the whales, porpoises, and dolphins. The majority 



live exclusively in salt water, but sonic enter the mouths of 

 rivers and may ascend them for a variable distance; others, 

 like the dolphins of the Amazon and the Ganges, are confined 

 t'o fresh water. The general shape of the body is fusiform, 

 more or less fishlike, covered with a thick, naked skin ; hairs 

 are confined to a few bristles about the mouth, and even these 

 may disappear in the adult. There is no externally marked 

 neck to separate the head from the trunk ; the anterior append- 

 ages have the form of flippers or paddles, while the posterior 

 are invariably absent. Often there is an unpaired median 

 dorsal fin, which, however, does not have a skeletal support 

 like that in the fishes. At the posterior end of the body the 

 tail is flattened and expanded horizontally into two lobes, the 

 "flukes." The head is often very large, in some cases being a 

 third of the total length of the body. The mouth is usually 

 very wide ; the eyes are relatively small ; the auditory opening 

 is very small, and there is no external ear. The external nasal 

 openings are on the top of the head ; in the whalebone whales 

 there are, as in other mammals, two external nares, but in all 

 the other Cetacea the two are united into one ; the nasal open- 

 ings in this order are often called spiracles or blowholes, since 

 the expired air carries aqueous vapor with it, so that when the 

 animal breathes it appears to blow water from its nostrils. 

 There are two mammary glands, which lie near the external 

 genital opening. 



Beneath the skin is a thick layer of fat, the "blubber," which 

 serves to maintain the high temperature of the both. The 

 bones are light and spongy, and so differ strikingly from the 

 bones of the Sirenia. Teeth when present are homodont and 

 Very variable in number in different species. The tongue is 

 thick, fleshy, and immovable. The lungs are very large, and 

 the thoracic cavity extends far back in the body. The testes 

 are abdominal in position and the uterus two-horned. The 

 Cetacea feed upon other animals; they include the largest 

 living animals and comprise about two hundred species, which 



