AVES. 397 



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. 



5. Some of the bronchial tubes terminate in air spaces (not 

 true lung tissue) located in various parts of the body. These 

 communicate with air cavities in some of the bones. 



6. The parts of the skeleton are much fused. There are no 

 teeth, the jaw being sheathed by a horny product of the epi- 

 dermis (beak). 



7. The right ovary and oviduct are aborted or rudimentary. 



8. All are oviparous ; yolk abundant ; segmentation dis- 

 coidal; amnion and allantois present. 



421. Form. The birds, like many of the extinct reptiles, 

 are bipeds. The axis of the more or less stout body makes an 

 angle of varying size with the axis of the legs, that is, the 

 vertical. The sacrum and the soft parts of the body project 

 behind this point of union in such a way as to balance the 

 anterior parts. The anterior appendages are not always well 

 developed but are much anterior to and above the centre of 

 gravity. This results in a more stable position of the body 

 in flight. The posterior appendages are relatively long, some- 

 times extraordinarily so. In all cases there is an interesting 

 correlation between the length of the neck and that of the legs. 

 The wading birds are especially endowed in these particulars. 

 The posterior appendages usually have four digits. These may 

 all be directed forward as in some swifts, or much more com- 

 monly the great toe (number i) is directed backward; in some 

 species two are turned backward and two forward. In swim- 

 ming birds a web is present which stretches from toe to toe. 

 The special form and arrangement of the web differ in dif- 

 ferent species. The digits end in claws which vary greatly in 

 accordance with the habits of the possessor. The anterior 

 appendages usually show traces of three much reduced digits. 



422. Supplementary Studies. Allow students to make a series of 

 studies of the angle made by the axis of the body with a vertical line 

 in various birds. Compare this angle in the robin when at rest and 



