39^ ZOOLOGY. 



membrane? Are the eyes movable? Do they view 

 the same field? 



Do the two together cover the entire field of view? 



How much external ear is present ? 



Are the scales homologous with feathers? (See refer- 

 ence texts.) 



DESCRIPTIVE TEXT. 



419. Birds must be looked upon as sharing with mammals 

 the first place in the animal kingdom. Even the mammals as 

 a class are not so highly specialized in structure and in habits 

 as the birds. Their most striking features of specialization are 

 connected with the demands of aerial life which they have so 

 successfully met. They share with the insects, the most 

 specialized of the invertebrate phyla, the most perfect de- 

 velopment of the power of flight found among animals. It 

 fpllows from their high degree of specialization that they are 

 among the most easily recognized of the vertebrates. The 

 earliest geological traces of birds show that they are closely 

 linked with reptiles in their origin, and the modern birds pre- 

 serve many interesting likenesses to the reptiles. Some of 

 these are seen in the scales on the shank and feet of birds ; in 

 the habit of laying large, well-nourished eggs which hatch out- 

 side the body; in the structure of the egg and its mode of 

 cleavage; the peculiarities of the ankle joint; in the presence 

 of a cloaca. 



420. General Characteristics of Birds. 



1. The Aves are vertebrates in which the epidermic out- 

 growths usually take the form of feathers (or scales in special 

 regions, as the feet). 



2. The anterior appendages in the majority of forms are 

 modified for flight. Associated with this is a large develop- 

 ment of the pectoral muscles and the bones to which they are 

 attached. 



3. A single occipital condyle. 



4. The heart is completely four-chambered; only one (sys- 



