144 ZOOLOGY 



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. 



178. Class Amphibians. In external appearance the mem- 

 bers of this class often look somewhat like reptiles, and they 

 have certain possessions in common with them, as the cold blood 

 and the three-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. 



179. Class 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. 



1 80. 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 (notockord; 

 see 349), 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 char- 

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

 blood has red corpuscles. They are called Chordata (having 

 notochord) or Vertebrata. This is the highest, best developed 

 phylum of the animal kingdom. The five classes which have 

 been mentioned are included in it. All other animals, with the 

 exception of a few which seem intermediate in some respects, 

 are classed as Invertebrates, and agree in general in the follow- 



