CLASS AVES. 93 



Recapitulation of Essential Chaeacters. — The 

 animal is an air-breather, and never possesses gills at any 

 time of its life. The two sides of the heart communicate 

 with one another (in most cases), and the body is always 

 supplied with a mixture of pure and impure blood. The 

 blood is cold. The skin usually develops horny scales. 

 The lower jaw is jointed to the skull by means of a 

 " quadrate bone," and the skull is united with the back- 

 bone by means of a single joint. The condition of the 

 limbs varies, but in no case are more than two pairs pre- 

 sent. These characters distinguish the class Reptilia as a 

 whole. 



CHAPTER XXIL 



CLASS AVES. 



The class Aves (Latin, avis, a bird) includes only those 

 most familiar and beautiful of animals, the Birds. Instead 

 of finding any difficulty in selecting an example which is 

 both common and at the same time exhibits the leadinc; 

 peculiarities of the class, it would not be an easy matter 

 to choose a common bird in which the more important 

 characters of the entire division are not present. We 

 shall therefore select as our type the domestic Goose, 

 or rather the Grey Lag Goose {Anser ferus) from which 

 the domestic breed is descended. 



Like all birds, the Goose is a genuine biped, supporting 

 its body, in standing or walking, exclusively upon its 

 hind-legs. As in the great majority of birds, also, the 

 fore-limbs are converted into wings, and are employed in 

 supporting the body of the animal in the air, or, in other 

 words, in flight. The fore-limbs are thus useless for 

 purposes of grasping, and all acts of this nature are per- 

 formed by the beak — that is to say, by the jaws ; though 



