4 VERTEBRATE ANIMALS OF THE UNITED STATES 



alternate with each other in frogs and toads, mainly under the stimulus 

 of light; the so-called chameleon of our southern States and many other 

 lizards have a still more remarkable range of color change. Very old 

 individuals in all the classes of vertebrates often show differences in 

 coloration and markings which are due to wear and the fading of colors 

 or loss or injury of parts. 



Internal Characters. — Characteristic of vertebrates, and often of 

 importance in classifying them, is the internal skeleton, which forms 

 the supporting framework of the body.. In the most primitive fishes 

 this framework is composed entirely of cartilage and the ligaments and 

 membranes which serve to bind its various parts together; in the 

 higher fishes and all the other vertebrates it is formed principally of 

 bone. The ratio of the cartilage to the bone that forms the skeleton 

 varies much among vertebrates in the classes above fishes, it being 

 the largest in the amphibians and the smallest in the birds. The 

 backbone, the main skeletal axis of the body, is a flexible segmented 

 column consisting of a succession of the similar disk-like or cylin- 

 drical vertebrae which grow around the unsegmented embryonic rod-like 

 notochord and are bound together by intervertebral ligaments. The 

 vertebrae forming the column may vary in number in the various classes 

 of vertebrates, ranging from 6 in certain amphibians to 300 in some 

 snakes; they fall into groups which correspond to the main subdivisions 

 of the body of the animal. The skull, or anterior portion of the axial 

 skeleton, is composed of two parts, the cranium, which encloses and 

 protects the brain and the organs of special sense, and the visceral 

 skeleton, which surrounds the mouth and forms the framework of the 

 face, the jaws, the tongue and the gills, where they are present. These 

 two portions of the skull, which protect and support such diverse organs, 

 develop quite distinct from each other, and are but loosely bound 

 together in fishes, and more or less so in all other vertebrates in which 

 the mouth is employed mainly in seizing the food and not in masticating 

 it. In the mammals, however, which masticate their food, the need of a 

 solid base to chew against has brought about a firm union of the vis- 

 ceral with the cranial portions of the skull, which is thus given the 

 compact character which distinguishes it. 



The other internal organs in vertebrates are usually of less impor- 

 tance in classification, although the entire inner structure of the 

 vertebrate body bears the characteristic stamp of the group. This 

 is especially true of the circulatory system — the ventrally situated 

 heart, the closed system of blood tubes and the red blood corpuscles. 

 The heart is made up primarily of two portions, an anterior and a pos- 



