52 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



aguae, specialization has gone so far that each cord has it own tunnel in 

 a divided oesophagus, as shown in Fig. 19. This neat arrangement 

 obviously prevents the ingestion of particles of any great size, and H. 

 aguae is presumably exclusively a microphagous feeder. R. temporaria is 

 more versatile, for it has another method at its disposal. If Fig. 17 or 

 18 is studied, it can be seen that the direction of the stream of water 

 changes suddenly at the posterior end of the pharynx. This would 



NOTOCHORD 



FOOD 



SEPTUM 



0.5 MM 



Fig. 19. Transverse Section of the CEsophagus of a Tadpole of Hypopachus 

 aguae Showing the Two Separate Channels for the Mucous Cords 



throw any large particles into the wide oesophageal funnel by centri- 

 fugal force, and these would be conveyed down the oesophagus by 

 cihary action. 



It has been suggested that the gill filters are food-collecting organs, 

 and so they are in a hmited sense. It seems to have been overlooked 

 that filters of any kind are always intermittently-operated structures, 

 and that, since they are not ciliated, there is nothing to move food 

 collecting on them to the oesophagus except the water currents, into 

 which the food is probably thrown at intervals by a process of back- 



