160 



THE DIGESTIVE SYSTEM 



The iirodeles show evidences of the formation of a secondary 

 extension of the primary tongue, and this consists of muscle, con- 

 nective tissue, and epithehum, together with glands that secrete a 

 sticky mucous substance used in capturing food. When at rest 

 the tongue of the frog lies along the mid-line of the mouth's 

 floor, being attached in front and free behind. It can be flipped 

 out, and when extended assumes a spoon-like form for catching 

 its moving prey. The upper and lower surfaces are covered by 

 stratified columnar epithelium. The epithelium of the upper 

 surface of the resting tongue is comparati\'ely thin, with many 



Fig. 99. 



-Photograph of the tongue of the frog. The upper portion shows the 

 glandular lower surface of the tongue. 



goblet cells and may be folded to form numerous papillae. The 

 lower surface has a great many simple or branched tubular glands 

 with dilated ends separated from each other by a minute amount 

 of connective tissue. (Fig. 99.) The superficial epithelium is of the 

 stratified columnar type, but the glands are lined with simi)le colum- 

 nar cells, mainly mucous in nature. Considerable diffuse lymphoid 

 tissue may be found below the epithelial surface, and occasionally 

 a lymphoid condensation resembling a nodule. In the connective 

 tissue below the mucosal covering are bundles of striated muscle 

 fibers. The main blood vessels and nerves follow a central course 

 through the tongue. 



Chelonia and Crocodilia have a fleshy secondary tongue capable 



