GENERAL PREVIEW OF THE ANIMAL KINGDOM. 137 



includes snakes, lizards, turtles, and crocodiles. The reptiles 

 have some features which indicate that they may be distantly 

 related to both birds and mammals, as well as to th$ next 

 class. This is an additional reason why the group of reptiles 

 is a difficult one to define. In general they may be recognized 

 by the fact that their bodies are covered by scales or plates 

 instead of hair or feathers. They always breathe oxygen from 

 the air, as do birds and mammals. They usually have only 

 three chambers to the heart whereas in the former groups 

 there are four. The blood is not constantly warm as in birds 

 and mammals. They lay eggs very much like those of birds. 



173. Amphibians. In external appearance the members of 

 this class often look somewhat like reptiles, and they have 

 certain possessions in common with them, as the cold blood 

 and the 3-chambered heart. They are especially noteworthy 

 from the fact that they begin life breathing oxygen from the 

 water as fishes do, and later in life lose their gills, acquire 

 lungs, and get their oxygen from the air, as do the reptiles 

 and higher forms. Amphibians include the frogs, toads and 

 salamanders. This is not a very important group in nature, 

 but is intensely interesting to the student of zoology because 

 it seems to be a connecting link between the air-breathing and 

 the water-breathing forms. 



174. Fishes. Fishes are characterized by the fact that 

 they breathe by means of gills throughout life. The body is 

 often scaly; the appendages are fin-like; the blood is cold, and 

 the heart has two chambers. They are beautifully adapted 

 to life in the water and are easily recognized. 



175. Vertebrates and Invertebrates. All the animals of 

 which we have thus far spoken agree in certain particulars. 

 They all possess a dorsal rod of supporting matter (notochord; 

 see 341), which is often surrounded by cartilage or bone (the 

 vertebral column). The nervous system in all of them is 

 dorsal to this rod and to the digestive tract, and is tubular in 

 character. The heart is ventral to the digestive tract and the 



