the duck's beak. 355 



mandible, as it is scientifically called, is furnished along its 

 edges with a row of curved horny projections, very like the teeth 

 of a comb, and each of them coming to a point. There are some 

 fifty or sixty of these teeth on each side, and they are regularly 

 graduated in size, being longest in the middle of the beak, and 

 becoming very short at either end. They are set diagonally, 

 with the tips pointing backwards. The edges of the lower 

 mandible are turned up in a sort of fold, on the outside of which 

 is a row of grooves corresponding with the teeth of the upper 

 mandible, and, like them, being set diagonally. 



These teeth and grooves would of themselves make a very 

 efficient filter, but they are further aided by the tongue. This 

 is thick, fleshy, and very mobile ; so much so, indeed, that when 

 the mouth is opened the tongue is automatically thrust forward. 

 The edges of the tongue are, like those of the mandibles, 

 furnished with a filtering apparatus. Instead, however, of being 

 horny and stiff like those of the mandibles, they are mem- 

 branous and exceedingly delicate. Indeed, in order to see them 

 properly, it is necessary to place the tongue under water, so 

 that the membranous filaments shall be floated apart instead of 

 clinging together by their own weight. 



The whole of this apparatus is abundantly supplied with 

 nerves, and is evidently a most exquisite instrument of touch. 

 The reader will now understand the peculiar movements of 

 a duck's beak while feeding. Although the bird can and does 

 eat solid food, such as barley, and, by reason of its superior 

 width of beak, will very much defraud the poultry in a yard 

 where ducks and hens are kept together, it is chiefly fitted for 

 extracting nourishment from water, and will find abundant 

 subsistence where a hen would die of starvation. 



When the beak is plunged into the water, the mandibles are 

 rapidly opened and shut, the tongue incessantly working back- 

 wards and forwards between them. Consequently, not only 

 are the solid parts of the water strained between the comb of 

 the upper beak and the grooves of the lower, but they undergo a 

 further sifting or filtering from the delicate fibrils which fringe 

 the edge of the tongue. 



Another familiar example of the Filter is to be found in the 

 jaw of the Greenland Whale. In this animal, as well as in its 



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