using the skin instead. Even the frogs and toads can long survive the removal of their lungs. Many species of this class aid respiration by swallowing air. Frogs and toads force the air into their lungs by a swallowing action, made necessary by the absence of ribs; and they can be suffocated, and turtles also, by holding their mouth open.
In the amphibians, we have a class of animals making use of four distinct means of respiration; three of them—skin, gills, and lungs—in about equal degree. It might, therefore, be supposed that their respiration should be more active and the aëration of their blood more complete than in other vertebrates. Nevertheless, quite the reverse is the truth, for the reason that any function is better performed when localized or "specialized."
This latter fact is illustrated in the reptiles, which have a circulation as incomplete as the frog, with respiration more active, although
Fig. 7.—Longitudinal Section of a Bleak. s, anterior; s', posterior portion of the air-bladder; œ,œsophagus; l, air-passage of the air-bladder. (After Semper.)
possessing only lungs. These are, of course, better developed in the reptile, where they are large sacs, having the interior surface increased by foldings, producing sacculi. Serpents have the left lung undeveloped, the right one forming a long, cylindrical sac capable of holding a large amount of air. By this means water-snakes are rendered buoyant, and fitted for long submergence. The last fact is also true of turtles.
Fig. 8.—Dipnoi {Lepidosiren annectens), one of the Mud-Fishes, using the Air-bladder as a Lung.
Lizards and crocodiles have two lungs, usually somewhat divided, and extending through the whole trunk. By their inflation the chameleon can give itself a plump appearance.
Reptiles have only a slight motion of the ribs, and are remarkable as a class for feebleness of respiration, considering that their lungs are
