AVES 401 



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 discoidal; 

 amnion and allantois present. 



430. 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, sometimes 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 commonly the great toe (number i) is 

 directed backward ; in some species two are turned backward and 

 two forward. In swimming birds a web is present which 

 stretches from toe to toe. The special form and arrangement 

 of the web differ in different 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. 



431. 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. Com- 

 pare this angle in the robin when at rest and when running. Make outline drawings 

 of the shank and toes of all the types of birds which can be found, and discuss the 

 differences in the light of the habits of the birds. Compare these with figures in 

 texts. Make figures of the varieties of webs found in the aquatic birds. 



432. Covering. The form of birds as outlined above is 

 much modified by the presence of feathers. They increase the 

 26 



