lO VERTEBRATE ANIMALS OF THE UNITED STATES 



Birds also depend largely upon sight, and this sense has attained an 

 extraordinary development in many species. The accurate vision 

 of birds during swift flight and over great distances, and the instan- 

 taneous accommodation of it to their rapid manoeuvres in the air, 

 seem to indicate that birds have the best eyes among vertebrates. 

 Hearing is also highly developed in birds; smell and taste, however, are 

 apparently in a backward condition. 



In mammals all the special senses have reached a very high develop- 

 ment. As the bird is essentially a creature of the air and must be able 

 to see far and accurately, so the mammal belongs on or near the ground 

 and in most cases finds its greatest security and advantage in its remark- 

 able sense of smell. But its habit of life, which gives the mammal the 

 most varied environment among vertebrates, makes all the special 

 senses important to it, and has brought about the high state of perfec- 

 tion which characterizes all of them. 



The muscular system of vertebrates has but few distinctive features. 

 The body or somatic muscles of fishes and salamanders, which, being 

 aquatic animals, move through the water by alternate to-and-fro 

 movements of the trunk and tail, have a strictly metameric arrangement 

 and longitudinal fibres, and the appendicular muscles of all vertebrates, 

 although not metameric, are directly derived from them; both are 

 striated in structure and voluntary in character. The body muscles 

 of terrestrial vertebrates, however, especially those whose locomotory 

 movements are carried on exclusively by the appendages, have mostly 

 lost their primitive character, although it is still indicated in several 

 of them, notably the rectus abdominis. The attachment of a muscle 

 to a bone by a single tendon, by which the action of all the fibres is 

 concentrated on a single point, is a peculiarly vertebrate character. 

 The visceral muscles, which are involuntary and, with a few exceptions, 

 unstriated, are present in the walls of the various tubular internal 

 organs to which they impart the peristaltic movements characteristic 

 of them. 



Distribution. — ^The United States covers an area of continental 

 extent containing the greatest possible variety of conditions which may 

 surround the lives of the animals living within it. Certain physical 

 features of this area strike the attention of the observer as of especial 

 importance. Of these the following may be mentioned; the great 

 extent and number of the fresh water lakes in the northern portion of 

 the country, the great river basins which characterize almost every part 

 of it and especially that of the Mississippi, the mountain systems in the 

 East and the West, the extensive coastal plains bordering the Atlantic 



