124 A COURSE ON ZOOLOGY. 



and fishes. That which has been said of each type ap- 

 plies to the whole class to which the type belongs, ex- 

 cepting, of course, the peculiarities of the species. We 

 will now review the distinguishing features of each of 

 these classes of vertebrates. 



I. All the mammals are beings whose organization 

 presents remarkable analogies with that of man. Their 

 double and complete circulation starts from a heart di- 

 vided into four compartments, of which the two on the 

 right are completely separated from those on the left. 

 The arterial blood cannot, therefore, mix with the venous 

 blood in any manner. Their respiration is aerial, and is 

 effected by the lungs. They have a constant and toler- 

 ably high temperature : they are warm-blooded animals. 

 We may add that they are viviparous, that is, the 

 young are at once produced alive, and that they nour- 

 ish their young with milk. This last peculiarity is the 

 cause of the name mammal, meaning an animal having 

 teats. Lastly, their bodies are usually covered with 

 hair. 



Examples of mammals : man, monkey, dog, cat, bat, 

 rat, sheep, goat, horse, cattle, seal, whale. 



II. Birds have a complete and double circulation like 

 mammals. They are warm-blooded animals. Their res- 

 piration is aerial and pulmonary, and they exercise this 

 function with an activity in perfect proportion to the 

 great exercise of force required by their ordinary mode 

 of locomotion. They are oviparous, their bodies are 

 covered with feathers, and their forelimbs are never 

 organized for walking. 



Examples of birds : eagle, chicken, parrot, turkey, 

 quail, stork, duck, canary, sparrow. 



III. Reptiles are cold-blooded animals ; their circula- 



