546 



ANURA 



[CH. 



hinged, slanting backwards instead of projecting directly downwards 

 as in Urodela. The tongue is fixed to the floor of the mouth in 

 front, but i free behind ; it can be rapidly thrust out of the inouth 

 by bending the posterior end forwards and it can be as rapidly 

 retracted. It is used to whisk the insects on which the animal 

 feeds into the mouth. 



Behind the eye is a circular patch of thin, tightly stretched skin. 



FIG. 267. Three diagrams illustrating the development of the procoelous ver- 

 tebral column in Anura. 



A. Stage in which basidorsals, basiventrals and intercalaries are separate. 



B. Stage in which the basidorsals have extended downwards to form the 

 main mass of the vertebra and in which the dorsal intercalary has formed 

 an intervertebral cartilaginous pad in which a synovial cavity is appearing. 

 The distal part of the basiventral has disappeared. 



C. Stage of the completed vertebra. 



1. Basidorsal. 2. Notochord. 3. Basiventral. 4. Dorsal inter- 

 calary. 5. Intervertebral pad. 6. Synovial cavity of cartilage. 

 7. Vestigial rib. Dotting, cross-hatching, etc., as in Figs. 232, 242 and 

 254. 



This is the ear-drum or tympanic membrane, which closes ex- 

 ternally the Eustachian pouch of the gullet. It is believed that 

 this pouch or tympanum is the remains of the first gill-cleft, the 

 spiracle of Elasmobranch fishes. Sound impinging on the ear-drum 

 is conveyed to the wall of the ear capsule by a slender cartilaginous 

 rod ensheathed in its middle part by bone, the so-called columella 

 auris. In the Urodela sound has to find its way as best it can 

 through the skin and muscle of the head to the auditory organ. 



