14 INTRODUCTION 



To the vertebrates belong all the most familiar animals such as 

 fishes, frogs, snakes and dogs. Vertebrates have certain structures 

 in common: 



I. Remarkable similarity in the three divisions of the body, 

 head, trunk, and tail. 



1. All vertebrates are bilaterally symmetrical, i.e. a plane passed 

 through an axis of the body will divide it into two equal halves. 



3. A supporting axis, the notochord, is found at some stage of 

 development in all forms, but is replaced in all higher vertebrates 

 by an axial skeleton. Both appendicular and axial skeleton are 

 internal. 



4. All vertebrates have two body cavities, the coelom and the 

 digestive tube. 



5. The nerve cord is hollow and dorsal to the alimentary canal. 



Class Mammalia (13,000 species). — Mammals are warm-blooded 

 animals with hair or wool covering the body, having a muscular 

 diaphragm which separates the chest from the abdomen. They 

 never have gills, but breathe by lungs. They have a well developed 

 and usually convoluted brain with many important association 

 tracts. While some mammals are adapted to aquatic and others 

 to aerial life, the majority are suited to life on land. Except in a 

 few of the lower forms we find that before birth young mammals 

 are closely attached to their mothers by a structure called the 

 placenta. In general, mammals are further advanced at birth than 

 in the lower classes of vertebrates. Mammary glands furnish 

 nourishment to the young until they are able to shift for them- 

 selves. Mammals range from the most highly developed form, 

 man, to the primitive duckbill platypus, which lays eggs and has 

 diffuse mammary glands. 



Class Aves (23,000 species). — Birds are unlike mammals, having 

 specialized in a quite different direction. They have a body tem- 

 perature ten degrees higher than that of the mammals, and are dis- 

 tinguished from all other animals by the presence of feathers. Their 

 highly developed wings and pectoral muscles, hollow bones, large 

 lungs, and air sacs adapt the majority of them to an aerial life, 

 although some forms like the ostrich are flightless. Birds are of 

 great economic importance in the extermination of insects, but they 

 are of aesthetic value since many of them are beautifully colored, 

 while others are sweet singers. 



Class Reptilia (5,000 species). — Reptiles differ more widely 



