AVES 401 



Are the scales homologous with feathers ? (See reference 

 texts.) 

 III. Internal Structures. (The pigeon is a good type for 

 anatomical study.) 

 The student knows what internal organs and systems of 

 organs to expect in vertebrates, and may well be require^ 

 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. Record both by description and drawings. 



Descriptive Text 



428. Birds may 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 perfect 

 development 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 shpw that they are closely 

 linked with reptiles in their origin, and the modem 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, yolk-ladened 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. Epidemic outgrowths usually in the form of feathers (or 

 scales in special regions, as the feet). 



2. Anterior appendages in the majority of forms modified 

 for flight. Large development of the pectoral muscles and the 

 bones to which they are attached. 



3. A single occipital condyle. 



4. Heart completely four-chambered; only one (systemic) 

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