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 membranous 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 backwards 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 congeners, the “whalebone,” or “baleen,” as it is more properly called, is so formed that it allows liquids to pass through it, while it retains solids. Feeding as it does upon small marine matters, it would starve but for the filtering power of the baleen, which enables the animal to take into its vast mouth the sea-water with its inhabitants, and to expel the water through the plates and fibres of the baleen, while retaining the animals. The process of filtering, as well as the structure of the baleen, is so familiar that it does not need further description. * * * * * WE will now proceed to another filter, which is used in the air, and not in water, namely, the Mouth-guard or Respirator of the fork-grinder.