THE FROG AND VERTEBRATES IN GENERAL 185 



In the Frog the mouth, or buccal cavity, which is the specialized 

 anterior end of the alimentary canal, is comparatively large and 

 contains several noteworthy structures. The highly developed, 

 muscular tongue lies in the center of the lower jaw and extends 

 from near the extreme anterior end to the posterior part of the 

 mouth. It is attached to the ventral wall of the mouth at the 

 anterior end only. As a result of this type of attachment, the free 

 posterior end of the tongue can be extended quite a distance out of 

 the mouth and thus made use of in the capture of Insects or other 

 small animal forms for food. The extension of the tongue is ac- 

 complished by rapidly filling a lymph space which lies beneath it. 

 Posterior to the tongue, on the ventral wall of the mouth, is a 

 raised, circular body which has a slit-like opening, the glottis, 

 leading from the mouth cavity into the larynx and then to the 

 lungs. Also, on either side near the posterior end of the tongue 

 of the male Frog of some species there is a small opening through 

 which air passes into the vocal sacs. The latter are known to act 

 as resonators and thus increase the volume of sound. (W. f. 107.) 



In the posterior part of the upper jaw on either side is the open- 

 ing of an eustachian tube which leads to the cavity of the middle 

 ear. Anterior to the openings of the Eustachian tubes is a pair of 

 olfactory openings (internal nares) which connect with the 

 external nares, and lying between are two groups of teeth, known 

 as the vomerine teeth. Numerous other very small teeth are 

 borne around the margin of the upper jaw. The teeth of the Frog 

 are not used as masticating organs, but function solely for holding 

 food which has been captured. The bony portion of the upper jaw 

 is enclosed by a fleshy upper lip. The latter is lacking in the lower 

 jaw, so that when the mouth is closed the lower jaw fits into a 

 groove, the sulcus marginalis, inside the upper lip. 



The buccal cavity ends posteriorly in the pharynx which soon 

 merges into the esophagus. The latter continues as a small, 

 undifferentiated tube through the anterior part of the coelom and 

 then enlarges, as has been noted, to form the stomach which lies 

 chiefly on the left side of the animal's body. The stomach gradu- 

 ally decreases in size toward the posterior end and curves some- 

 what toward the right side of the body. 



The small intestine consists of two portions : namely, the an- 

 terior portion, known as the duodenum, which connects with the 

 pyloric end of the stomach ; and the posterior portion, known as the 



