Fig. 1. Lateral view of Terrapene carolina illustrating the two main lung ventilation mechanisms of turtles. Half the shell has been removed to reveal the internal morphological relationships between the lungs, abdominal muscles and skeletal elements. (A) Illustration of the abdominal muscles and lungs of T. carolina. The paired transverse abdominis (TA) muscles wrap around the posterior portion of the lungs and produce exhalation by compressing the lungs as they contract. The cup-shaped oblique abdominis (OA) muscles actively produce inhalation as they flatten and expand the inguinal flank postero-ventero-laterally. (B) Photograph of the skeleton with limbs and neck fully extended. Because the shell contains a fixed volume, the lungs can be filled with air when the head and limbs are protracted. (C) Air can be forced out of the lungs when the limbs and head are retracted into the shell. Our recordings show that when T. carolina is in this fully retracted position, some air remains in the lungs and breathing is possible with the use of the abdominal muscles.

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