. The biology of the amphibia. Amphibians. 334 THE BIOLOGY OF THE AMPHIBIA Salientia, although a few have the integument unmodified and separable from the underlying drum, and many burrow- ing or aquatic types as well as all the urodeles lack both drum and middle ear. This might be considered evidence that the urodeles sprang from aquatic or burrowing ancestors. Amphibia were primitively equipped with a tympanum which was fully exposed on the side of the head. It is curious that in one frog (Rana cavitympanum of Siam), the tympanum has sunk below the surface and lies at the end of an external
. The biology of the amphibia. Amphibians. 334 THE BIOLOGY OF THE AMPHIBIA Salientia, although a few have the integument unmodified and separable from the underlying drum, and many burrow- ing or aquatic types as well as all the urodeles lack both drum and middle ear. This might be considered evidence that the urodeles sprang from aquatic or burrowing ancestors. Amphibia were primitively equipped with a tympanum which was fully exposed on the side of the head. It is curious that in one frog (Rana cavitympanum of Siam), the tympanum has sunk below the surface and lies at the end of an external ear opening as in mammals. In the first land vertebrates as well as in their immediate ancestors, the support of the lower jaw was shifted from the. Fig. 121.—Diagram of the sound transmitting apparatus of an aquatic larval (A) and a terrestrial adult salamander (B). Col., columella; , fenestra vestibuli; , hyoid arch; , ligamentum squamoso-columellare; , skeleton of the lower jaw; , musculus opercularis; Op., operculum; Pq., palatoquadratum; Sq., os squamosum; , suprascapula; , stilus columellae. (After Kingsbury and Reed.) hyomandibular to the quadrate. The freed element sank into the spiracular cavity and assumed a new function of transmitting sound vibrations from the tympanum to the ear capsule. In Eogirinus, this hyomandibular, now called a " columella," merely abutted against the ear capsule, but very early in laby- rinthodont phylogeny a fenestra was formed which increased the efficiency of transmitting vibrations to the perilymph, the fluid surrounding the membranous labyrinth or inner ear. Modern Amphibia show in their ear apparatus some evidence of terrestrial ancestry. In addition to the columella which has its platelike proximal end fitting into the fenestra vestibuli, there is present primitively in both frogs and urodeles a second bony or cartilaginous plate in the same fenestra. This element,. Please note that these imag
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