CHAPTER XXV. 



CHORDATA (CONTINUED): CLASS AVES (BIRDS). 



324. General Statement. Birds share with mammals 

 the first place in the animal kingdom. They share with 

 insects the most specialized of the invertebrate branches 

 the highest development of the power of flight found 

 among animals. Add to this their great variety of color, 

 their great abundance, and their interesting diversity 

 of habits and instincts, and we have the most attractive 

 group of animals in the whole animal series to the general 

 student and lover of nature. 



Birds are the most easily recognized of all the animals. 

 This means that the structural differences among them 

 are relatively slight. They do differ greatly in their 

 habits of life, however. Some, as the ostrich, cannot 

 fly and only run about on the land ; others, as the penguins, 

 cannot fly, but are excellent swimmers and live on or 

 about the water; most can fly well and may frequent 

 either land or water. 



Notwithstanding the ease with which they may be 

 recognized, the birds have some interesting likenesses to 

 the reptiles. They have scales on the shank and feet; 

 they lay large, well-nourished eggs, which hatch outside 

 the body; the digestive tract and the excretory and 

 reproductive organs open into a common chamber (cloaca) , 

 and this communicates with the outside by a single open- 

 ing; the structure of the ankle-joint is very similar in the 

 two groups. Furthermore, the early geological birds 



