456 



Comparative Morphology of Chordates 



Epidermis 



Annulus tympanicus 



Columella 



Pharynx 



Basihyoid 



Tympanic 

 cavity 



Endoderm 



Mandible 



Fig. 353. Auditory apparatus of an ainiran. The stereogram exposes a narrow 

 vertical band of the animal's sagittal plane and the left half of a transverse section 

 which has been swung several degrees backward on its sagittal axis to bring it into 

 a favorable angle for the observer. Both sections cut through the pharynx. The 

 transverse section bisects the tympanic membrane and shows the left auditory nerve 

 (VIII) passing from the medulla oblongata to the internal ear (membranous 

 labyrinth ). The sagittal view shows the forked anterior tip of the retracted tongue 

 pointing backward into the pharynx. 



ally procoelous (concave only on the anterior face) or opisthocoelous 

 (concave only on the posterior face) or may have some modification of 

 these forms. 



The legs are strongly developed — forelegs short; hindlegs much 

 elongated and adapted to leaping locomotion, having, instead of the 

 usual three segments (thigh, shank, and foot), a fourth interposed by 

 great elongation of two bones of the ankle (Fig. 345). 



The ear in Anura possesses an accessory mechanism not found in 

 other modern amphibians or in fishes. In the posterior region of the 

 head there is, on each side, a cavity which internally opens into the 

 pharynx and externally is separated from outside space only by a thin, 

 taut tympanic membrane which, as viewed from outside, appears 

 as a nearly circular flat area of skin just behind the eye (Figs. 352. 353 ) . 

 A slender rod of bone, the columella, extends across this cavity 

 (tympanic cavity), its outer end being attached to the inner surface 

 of the tympanic membrane and its deeper end attached to a very small 

 bone (stapes) which is inserted into that part of the wall of the 

 cranium within which the "internal" or sensory ear is lodged. Vibra- 

 tions of the external medium impinge upon the tympanic membrane 

 and are transmitted by the columella to the internal ear. In fishes the 

 ear is probably more important as an organ of equilibrium than as an 



