400 ZOOLOGY 



to block out an outline of work. What are the principal 

 sets of organs to be expected? How do these compare 

 with the corresponding structures in the fish or frog ? Do 

 you find any structures not found in the lower types. Un- 

 certainties may be settled by reference to more extended 

 texts. 



DESCRIPTIVE TEXT 



428. 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 

 follows 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 outside 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. 



429. 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 development 

 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- 

 temic) aortic arch which turns to the right; red corpuscles oval 

 and nucleated. High bodily temperature, 100 to 110 F. 

 Renal portal circulation almost wanting. 



