412 GENERAL ZOOLOGY 



roll themselves into a ball, thus offering a hard surface in 

 every direction. 



The sloths of South and Central America are nocturnal, 

 arboreal animals; their natural attitude during the day is 

 hanging from a branch back downward. The hair is gray, 

 but in some species it offers a lodgment for a green alga, a 

 plant of low organization, which gives the hair a green tinge 

 like that of the masses of vegetation of the tropical forest. 

 Sloths rarely descend from the branches of trees. 



Whales, Dolphins, Porpoises, and Allies. The whales, dol- 

 phins, and porpoises are true mammals, though so adapted 

 to their aquatic life that they seem, on superficial examina- 

 tion, to be fishes. The name of the order, Ceta'cea, is derived 

 from the Latin cetus, a whale. 



Flower and Lydekker, in their Mammals, thus review the 

 principal peculiarities of the group : " The external fish-like 

 form is perfectly suited for swimming through the water ; 

 the tail, however, is not placed vertically as in fishes, but 

 horizontally, a position which accords better with the con- 

 stant necessity for rising to the surface for the purpose of 

 breathing. The hairy covering characteristic of all mammals, 

 which, if present, might interfere with rapidity of movement 

 through the water, is reduced to the merest rudiments, a few 

 short bristles about the chin or upper lip, which are often 

 present only in very young animals ; and the function of 

 keeping the body warm is performed by the ' blubber.' The 

 fore limbs, though functionally reduced to mere paddles, 

 with no power of motion except at the shoulder-joint, have 

 beneath their smooth and continuous covering all the bones, 

 joints, and even most of the muscles, nerves, and arteries of 

 the human arm and hand ; the rudiments of hind legs, found 

 buried deep in the interior of the animal, apparently sub- 

 serve no useful purpose, but point an instructive lesson to 

 those who are able to read it." 



