xii. 23 FEEDING 343 



to prevent the escape of the prey; few amphibia bite. Biting teeth 

 are present, however, in the adult Ceratophrys ornata, whose larvae 

 also have powerful jaws and are cannibalistic. The South American 

 tree-frog Amphignaihodon has teeth in the lower as well as the upper 

 jaw and presumably has redeveloped them, a remarkable case of the 

 reversal of evolution. Teeth are also present on the lower jaw of most 

 urodelcs. 



dhc 



Fig. 201. Transverse section of the spinal cord of a frog, showing cells in the grey 

 matter with their axons and dendrites spreading into the 'white' matter. 



ax. axon of ventral horn cell, leaving cord in ventral root; d. dendrite of ventral horn cell; 



dhc. small cells of dorsal horn; dr. dorsal root entry; m. cell body of ventral horn cell; tieur. 



neuropil at periphery of spinal cord; vhc. small cell of ventral horn. (After Gaupp.) 



The oesophagus is not sharply marked off from either mouth or 

 stomach and the latter is a simple tube. Its lining epithelium of 

 mucus-secreting cells is folded and simple tubular glands open at the 

 base of the folds. These glands, unlike those of mammals, are com- 

 posed of only a single type of cell, which secretes both the acid and the 

 pepsin found in the stomach. 



The intestine is marked off from the stomach by a pyloric sphincter. 

 It is relatively short and dilates into a large intestine, there being a 

 valve interposed in the frogs, though not in all amphibia. The liver 

 and pancreas have the structure common to all vertebrates and pro- 

 duce juices of the usual type. The intestine of the omnivorous tadpole 

 is more coiled than that of the adult frog. The type of food taken 

 depends on what is available, most species of amphibia are not par- 

 ticular feeders. However, they can learn with only one or two trials 

 to avoid distasteful insects. Frogs and toads devour large numbers of 

 insects. If the common insects available are pests the amphibian's part 

 in controlling their number works to the advantage of man. 



