. Animal life as affected by the natural conditions of existence. Animal ecology. 8 INTRODUCTION. In order to do thia, we will return to the more striking of the examples here given. There can certainly be no doubt of this—that the lungs of mammals have not been developed by modification from those of land snails; we know on the contrary, or will assume, that the luligs of all the Vertebrata are identical, and to be regarded as. modifications of the air-bladders of the bony fishes (fig. 3), although these organs do not serve, or at any rate do not mainly serve, for respiration. On the contrary
. Animal life as affected by the natural conditions of existence. Animal ecology. 8 INTRODUCTION. In order to do thia, we will return to the more striking of the examples here given. There can certainly be no doubt of this—that the lungs of mammals have not been developed by modification from those of land snails; we know on the contrary, or will assume, that the luligs of all the Vertebrata are identical, and to be regarded as. modifications of the air-bladders of the bony fishes (fig. 3), although these organs do not serve, or at any rate do not mainly serve, for respiration. On the contrary, fish breathe by their gills. But the lungs of mammals difier remarkably in structure from those of birds, and yet more from. Fio. i.—a, the bouo of a Cat, showing the marrow-tube; b, that of a Bird with cavities containing air instead of marrow; c, the skull of a Buceros sawn tiirough. Air-cavities traverse every patt of the bone. ' ' ^ . r- - those of the lower reptiles or the amphibia. In these last they are usually simply capacious sacs opening into the mouth by a very short passage (the trachea); in mammals they exhibit a spongy structure, and often a highly complicated arrangement of extremely long air-tubes; in birds also the lungs have a spongy structure, and connected with them there are always numerous air-cavities which lie partly in the cavity of the body and partly, in the form of canals, deep in the bones of the skull (fig. 4) and of the vertebral column, or penetrate to the end of the extremities, forming what are known as pneumatic bones. Now these differences in the structure of the lungs of. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Semper, C. (Carl), 1832-1893. New York, D. Appleton
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