146 ZOOLOGY 



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 

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

 birds. 



178. Class Amphibia. — 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 blooded- 

 ness and the three-chambered heart. They are especially note- 

 worthy from the fact that they begin life usually 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 Pisces. — Fishes are cold-blooded animals charac- 

 terized by the fact that they breathe by means of gills through- 

 out life. The body is often scaly; the appendages are fin-like; 

 and the heart has two chambers. They are beautifully adapted 

 to life in the water and are easily recognized. 



180. 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 §349), which is often surrounded by cartilagijor 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). 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 following facts : — there 



