IV. VERTEBRATA: MAMMALIA, CETACEA 

 Order X. Cetaceao 



571 



In form the whales resemble the sirenians, a result of an aquatic life, but the 

 resemblance ends here. The whales are so fish-like that every one speaks of 

 the whale fishery. Head and trunk are scarcely distinguished, the cervical 

 vertebras being very short and more or less completely fused. The hind limbs 

 are absent, of the pelvic girdle only a small ilium remains, and no sacral vertebras 

 are developed. The caudal fin differs from that of a fish in being horizontally 

 flattened; the skin is thick and is sparsely haired or completely naked, some 

 lacking hair even in the embryo. Most of the species inhabit the high seas, 

 Inia boliviensis and Platanista gangetica occur in rivers. 



The fore limbs are modified into flippers, the bones of which are of nearly 

 equal size and are jointed only at the shoulder. A dorsal fin ('fin backs') occurs 

 in some. The lack of hair is compensated by the thick layer of subcutaneous 

 fat (blubber). In order that the animals may breathe while feeding, the larynx 

 is prolonged into a tube which extends up through the pharynx to the choanae, 

 from which the nostrils extend directly upwards to the single (Denticetes) or 

 paired (Mysticetes) external opening. Since the air expelled contains much 

 moisture and this is condensed on contact with the cooler external air, the im- 

 pression was natural that the animals in 'blowing' spouted water. As the 

 olfactory membrane is degenerate and the olfactory lobes are reduced, the nose 

 is an organ of respiration only. 



FIG. 619. Section through jaws of whalebone whale (after Delage). r, septum 

 of nose; m, mouth cavity; mx, maxillary bone; p, premaxillary (hinder end); v, vomer; 

 iv baleen. 



The eyes are small, external ears are lacking, the mammae are close to the 

 sexual opening. The teeth are either present in large numbers, similar and 

 conical, and, since the second dentition is rudimentary, are monophyodont 

 (Denticetae) or they are outlined early and then resorbed and replaced by plates 

 of baleen (Mysticetae), composed of large horny plates (whalebone) in large 

 animals a dozen feet long (fig. 619, w), of which hundreds are arranged in close 

 succession extending inward to the tongue. They correspond to the transverse 

 palatal folds which occur in other mammals. As they are fringed on the inner 

 edges they form a strainer which retains the small animals on which these 

 whales feed. The origin of the whales is one of the unsolved problems. 'I hat 

 they had a terrestrial, quadrupedal ancestry is beyond question, and the little 



