402 ZOOLOGY 



aortic arch, which turns to the right; red corpuscles oval and 

 nucleated. High bodily temperature, ioo° to iio° F. Renal 

 portal circulation almost wanting. 



5. Some of the bronchial tubes terminating 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. Parts of the skeleton much fused. Teeth absent, the jaw 

 being sheathed by a horny product of the epidermis (beak). 



7. Right ovary and oviduct aborted or rudimentary. 



8. Oviparous; yolk abundant; segmentation discoidal; am- 

 nion and allantois present. 



430. Fonn. — 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. — 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 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. 



