CLA WS. 



173 



is a layer of cellular tissue containing a similar fluid, which seems 

 to be connected with the growth of the feathers. 



The feet of Birds are as varied in different species as are their 

 wings. In Birds of Prey the claws are powerful and hooked ; in 

 others the foot is flat, claws straight and adapted for walking (Figs. 55, 



Fig- 55- 

 Foot of White-headed Eagle. 



Fig. 56- 

 Foot of Honey Buzzard. 



Fig. 57- 

 Foot of Cassowary. 



56, 57). The great toe is generally the strongest, but this is not 

 an absolute law : a projection which is found on the leg of some 

 species is designated a spur, and is a formidable weapon (Figs. 58, 59). 

 Some birds walk by bringing their feet forward alternately ; others 

 by a simultaneous motion, resembling a succession of leaps. Some 



^v 



Fig. 59.- Foot of Pheasant. 



run with great velocity, while others walk with great difficulty, and 

 then only on a flat surface. Many have their toes joined by thin 

 membranes, which act as paddles by which they propel themselves 

 through the water. 



The beak or bill of Birds is composed of two bony pieces, 

 called mandibles, surrounded by a horny substance, differing both in 

 form and thickness, according to the habits of the species. In the 



