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18) The bodies of birds are adapted for flying. Many of a bird's bones are less dense than human bones, which makes birds' bodies lightweight. Flying birds have large chest muscles that move the wings. ... The air sacs enable birds to extract much more oxygen from each breath of air than other animals can.

19)The wings of a cockroach are external appendages attached to the prothorax and the mesothorax. ... The wings of the bird are actually modified forelimbs, attached to the pectoral girdle, to which feathers are attached.

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