CLASSIFICATION OP THE ANIMAL KINGDOM. 243 



anterior extremities of whales into broad fins and paddles, enabling them 

 to move through the waters of the sea with as much ease as the most 

 perfectly organized fish; the elongation of the metacarpus of the horse; 

 the remarkable shape of the skeletons of the sloth tribe, adapted to their 

 existence amongst the branches of trees; the elongation of the fingers of 

 bats, whereon their membranous wings are stretched; and many others, the 

 enumeration of which would occupy too much space. 



The transition from birds to quadrupeds, says Professor Jones, is effected 

 by gentle gradations of structure, and the Monotremata, of which we have 

 a familiar example in the Duck-billed Ornithorhynchus paradoxus of New 

 Holland, forms the lowest connecting link. The muscular system of mam- 

 mals is in general very variable and highly constructed. One of its dis- 

 tinguishing features is the diaphragm or muscle, which divides the thoracic 

 from the abdominal cavity, and is peculiar to the entire class. Another 

 important one is the cutaneous muscle for moving the integument, which, 

 in some species, as the porcupine and hedgehog, is very highly developed. 

 In man this muscle is found only partially adapted to different offices of 

 the body. Their digestive system is very variable and complex; the teeth, 

 which arm the mouth, and seize and masticate the prey, are a study in 

 themselves alone. Those of whales, forming the whalebone of commerce, 

 are of a horny texture, and hang in fibres from the roof of the mouth, 

 straining the waters of the ocean as through a sieve, and catching thousands 

 of mollusca and other animals in their net-like fringe. The male narwal, 

 instead of the usual teeth planted round the jaw, possesses a single tusk 

 of great strength, attaining sometimes the length of eight or ten feet. The 

 tusks of the well-known elephant offer another remarkable modification of 

 the dental apparatus, and consist of ivory without enamel. But in far the 

 greater number of quadrupeds the teeth present a more complex structure, 

 consisting of two distinct substances — ivory and a coating of very hard 

 enamel; some continue to grow during the life of the animal, as the incisor 

 teeth of rodents; while others are limited in their growth, as those of man, 

 the carnivora, and others. 



The tongue, the seat of the sense of taste, corresponds in all mammals 

 in general muscular structure to that of man, with the exception of the 

 ant-eater and others which use it as a means of capturing insects, and 

 have it adapted accordingly to the purpose. In some animals this organ 

 is studded with recurved points; in the porcupine it is armed at its ex- 

 tremity with sharp horny scales. The salivary system in all mammals, 

 where it exists, corresponds generally with that of man. In the amphibious 

 genera it is but feebly developed, and in the cetacea it is not found at 

 all. The oesophagus is a long muscular tube, with a loose lining mem- 

 brane, capable of great distension; and the stomach, which is of endless 



