OR, PLAIN 



toes flattened and fringed, as the 

 phalarope, 17, and the grebe, 18. 

 By this provision they can walk 

 among the weeds without diffi- 

 culty, and when swimming, by 

 the overlapping of the fringes of 



399. 



their toes, they have the advan- 

 tages of web-like feet. Birds that 

 swim and dive, and walk chiefly 

 on muddy banks, are web-footed, 

 as the northern diver, 19. Those 

 that have large and heavy bodies, 

 and sit erect on the land, are 

 provided with large thick webbed- 

 feet, resembling the fins of the 

 seal, 20. The bodies of water-birds 

 are boat-like in shape, and the 

 ribs of divers are well developed, 

 to protect their internal organs 

 from the pressure of water. 

 Their plumage is thick and water- 

 proof, having underneath an oily 

 down which resists water. Many 

 use their wings, which are too 

 small for flight, as paddles, and 

 their tails act as rudders when 

 diving. 



The bones of birds are hollow, 

 their cavities being filled with 

 air, which passes into them 

 through small orifices which com- 



TEACHING. Ill 



municate with the numerous air 

 cells of the body. In large and 

 apparently heavy birds, which 

 possess great powers of flight, 

 these air cavities are found in 

 almost every bone ; whilst in 

 those whose habits are terrestrial, 

 nearly all the bones are destitute 

 of air cells. The bones of young 

 birds, which have not yet ac- 

 quired the power of flight, are 

 filled with marrow, which dis- 

 appears when they take to the 

 use of their wings. The forms 

 of their skeletons bear a wise 

 adaptation: to their habits. To 

 the various bones names are 

 given, by which they may be 

 distinguished in written, or oral 

 description. The cranium, 2, 

 the bones which enclose the 

 brain ; the mandibles, 3, the pair 



cc 



400. 



of jaws of which the beak is an 

 extension ; and in the upper one 

 of which the nasal fossal, 2a, or 

 cavity of the nose is situated ; 

 the cervical vertebra?, 4, or spine- 

 bones of the neck ; the sacral 



