DIVISION OF THE VERTEBRATA INTO CLASSES. 19 



22. The apparatus of digestion, in this great division of the 

 animal kingdom, offers but very slight differences. 



23. The blood is always red, and circulates in vessels called 

 arteries and veins. It is always set in motion by a fleshy heart; 

 but the conformation of this last organ, as well as the route of 

 the blood in the circulatory system, varies in the different classes 

 of this branch. 



24. Respiration always takes place in an apparatus situated 

 in an internal cavity of the body, but it is not always aerial as in 

 man ; it is sometimes aquatic, and then the lungs are replaced 

 by branchiae. 



DIVISION OF THE BRANCH OF THE VERTEBRATED 

 ANIMALS INTO CLASSES. 



25. Nature seems to have followed the same general plan in 

 the creation of all vertebrated animals, but nevertheless, these 

 beings differ from each other, and some of the differences which 

 they present, are of great importance in the exercise of life. 



26. Therefore, that the classification of the vertebrated ani- 

 mals may be, in a manner, a picture of the modification of their 

 organization, it becomes necessary to establish amongst them, 

 four grand divisions, called classes, which are named as follows : 



MAMMALIA, 

 BIRDS, 

 REPTILES, 

 FISHES. 



27. The following table presents at a view, the principal 

 characters of these classes : 



22. What is remarked of the apparatus of digestion in this great division 

 of the animal kingdom ? 



23. What is the color of the blood in the vertebra ta ? How does it circu- 

 late ? How is the blood set in motion ? Is the conformation of the heart 

 and the route of the blood in the circulatory system always the same in the 

 different classes of the branch of vertebrata ? 



24. How does respiration take place in the vertebrata? Is it always 

 aerial as in man ? When respiration is aquatic, is it effected through the 

 medium of lungs? 



25. In what respect are the differences presented by vertebrated animals 

 important ? 



26. How many classes compose the branch of vertebrata ? What are 

 these classes called? 



27. (KTQuestions answered by the table.') What are the characters of ani- 

 mals of the class mammalia? How many cavities-has the heart? Is the 

 blood warm or cold in animals of this class ? 



What is the character of the respiration in the class of birds ? Is the 

 blood cold in this class of animals ? How many cavities has the heart ? 



