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 shape to enable the body to penetrate the air, and a small neck would destroy the conical form. The internal organs are compactly arranged and rest in the cavity of the breast bone. The bellows-like air sacs filled with warm air lighten the bird's weight. The bones are hollow and very thin. The large tail quills are used by the bird only in guiding its flight up and down, or balancing on a limb. The feet also aid a flying bird in balancing. The wing is so constructed as to present to the air a remarkably large surface compared with the small bony support in the wing skeleton. Are tubes ever resorted to by human architects when lightness combined with strength is desired? Which quills in the wing serve to lengthen it? (Fig. 296.) To broaden it? Is flight more difficult for a bird or a butterfly? Which of them do the flying machines more closely resemble? Can any bird fly for a long time without flapping its wings?

(Order?)

Exercise in the Use of the Key.—Copy this list and write the name of the order to which each of the birds belongs. (Key, page 177.)

Cockatoo (Fig. 320) Sacred Ibis (Fig. 328) Screech Owl (Fig. 311) Nightingale (Fig. 325) Top-knot Quail (Fig. 329) Wren (Fig. 310) Apteryx (Fig. 318) Lyre bird (Fig. 327) Road Runner (Fig. 313) Ostrich (Fig. 332) Penguin (Fig. 330) Pheasant (Fig. 319) Wood Duck (Fig. 314) Jacana (Fig. 324) Sea Gull (Fig. 322) Heron (Fig. 315) Hawk (Fig. 312)