CHAPTER XXVI 

 AN INTRODUCTION TO THE VERTEBRATES 



The animals that remain to be discussed all belong to the 

 phylum of back-boned creatures, the Vertebrata. There are 

 only about thirty thousand species of these as compared with 

 over four hundred thousand Invertebrates, but their large size 

 and intimate relations with man make them of comparatively 

 greater importance. Unlike the invertebrates, the vertebrates 

 are well known to us, although there are many of them that never 

 come within our ordinary, everyday experiences. The verte- 

 brates may be divided into five classes, which are as follows, 

 beginning with the lowest forms: — 



Class i. Pisces. — Fishes (Fig. 138, A and B). 



Class 2. Amphibia. — Frogs, Toads, and Salamanders (Fig. 

 138, C and D). 



Class 3. Reptilia. — Lizards, Snakes, Crocrodiles, and Tur- 

 tles (Fig. 138, E and F). 



Class 4. Aves. — Birds (Fig. 138, G). 



Class 5. Mammalia. — Hairy Quadrupeds, Whales, Seals, 

 Bats, Monkeys, and Man (Fig. 138, H). 



Most of us have seen examples of all these different classes 

 of vertebrates. The horse is a typical mammal; the hen or 

 pigeon a typical bird; the snake, turtle, and crocodile typical 

 reptiles; the frogs and toads typical amphibians, and fish are, of 

 course, a common article of food. Although we thus have an idea 

 of the different types of vertebrates, they are mostly domesti- 

 cated animals, and we do not see very many different kinds of 

 wild vertebrates unless we are fortunate enough to live where 

 there are aquaria containing fish and other aquatic animals, or 



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