236 BIOLOGY FOR BEGINNERS 



this very fact necessitated an active and intelligent life to oppose 

 or escape their enemies. The vertebrates thus have come to be 

 the highest in the scale of animal development and include the 

 following classes: 



1. The Pisces (fishes). 



2. The Amphibia (frogs, toads, salamanders). 



3. The Reptiiia (snakes, turtles, lizards). 



4. The Aves (birds). 



5. The Mammals (rat, cattle, cat, man, etc.). 



The vertebrates include many very different animals, but they 

 all agree in the following points, in which they also differ from all 

 the other forms studied. These other forms are sometimes all 

 classed together as the invertebrates. 



All vertebrates have, 



1. An internal skeleton of bone or cartilage. 



2. A spinal column composed of vertebrae. 



3. A dorsal nervous system. 



4. Two body cavities: a dorsal one for the nervous system and 



a ventral one for the other organs. 



5. Eyes, ears, and nostrils always on the head. 



6. Jaws, not modified limbs; move up and down. 



7. Eyelids and separate teeth are usually present. 



8. The heart is ventral and blood is red. 



9. Never more than two pairs of limbs. 



The human body is a true vertebrate type as we can see by 

 comparing its structure with the above points and we only hold our 

 place in the race of life by our superior brain development. There 

 is not one of the lower groups but has members which excel us in 

 other respects. 



Compare our swimming with the fish, our flight with the bird, 

 or our speed with the deer and it will be seen that we are inferior 

 in many respects to the different members of the animal kingdom. 

 It is the development of our brain that has enabled us to retain 

 the lead in the race of life. Superior intelligence compensates 

 many times over for various physical disadvantages. 



