Oxygen..

[Anomalocaris]

Another trivia, the gas exchange in a chicken happens inside its bones, abdominal cavity, as well as in its lungs. If you nick the main wing bone of a chicken, you can tape up the chicken's nose and beak, and it can still breath.

Seems an awful lot of work to go through, not to mention the chicken may have had something planned for later that day.

Boru

Well, I always found the respiratory systems of birds an awe inspiring organ.