CETACEA. 



size, that only water can 

 carry them, and the sea 

 supply them with food. The 

 cervical region is not visible 

 externally, and the hmd 

 passes directly into the cylin- 

 drical trunk, while the caudal 

 end develops a horizontal fin, 

 in addition to which there 

 is often a fatty fin on the 

 dorsal surface. Hairs are 

 almost entirely absent in the 

 larger forms ; being only re- 

 presented by bristle-like 

 hairs on the upper lip, which 

 are present during the whole 

 of life, or only during the 

 foetal period. In the smaller 

 species, and in the Sirenia 

 there is a sparse covering of 

 bristles. On the other hand, 

 there is developed beneath 

 the thick leathery skin in 

 the subdermal cellular tissue 

 a considerable layer of fat, 

 which to a certain extent 

 takes the place of fur, and 

 serves both to prevent the 

 loss of heat and to lower 

 the specific gravity. The 

 head is often elongated into 

 the form of a snout, and is 

 without an external ear. 

 The eyes are strikingly small 

 and are often placed near the 

 angle of the mouth; the nasa 

 apertures are shifted on to 

 the forehead. The anterior 

 limbs are represented by 

 short, externally unjointed 

 swimming fins, which can 



