FOOD AND FEEDING 91 



allowing water taken in at the mouth to pass out, while retain- 

 ing the small particles of food taken in with each gulp. The 

 expulsion of the water is performed by raising the tongue, 

 which, as in all the ducks, and the "whale-bone" whales, is 

 extremely large and fleshy. In fish-eating ducks, such as the 

 merganser, the beak is long and narrow, while the plates along 

 the edges of the beak take the form of conical, horny teeth well 

 calculated to hold a slippery prey. In some other fish-eating 

 birds, however, as in the darter, the edges of the beak are armed 

 with needle-like points, and in yet others these edges are serrated 

 as in the gannet. The herons, which are fish eaters, have a 

 dagger-like beak. Birds like the woodcock and snipe, which 

 obtain their food by probing in mud, have the beak of great 

 length, and probe-like ; but so constructed that the tip of the 

 upper jaw can be raised when buried deep in the soil. Being 

 richly supplied with nerves, it further acts as an organ of touch, 

 so that when it comes in contact with a worm the tip of the 

 jaw is raised just far enough to grasp the worm and press it 

 against the lower jaw, when it is withdrawn. But for this con- 

 trivance it would be necessary to thrust aside the whole mass 

 of mud opposed to the buried beak, a task too great for so slender 

 an organ. Humming-birds have a similar style-shaped beak, 

 but this is used for a quite different purpose ; inasmuch as it is 

 thrust up into the tubular corollas of flowers in search of insects 

 which are drawn into the mouth mixed with honey by means 

 of the tongue, which has a curious and complicated structure 

 contrived to effect this purpose. In some birds, as in the 

 spoonbill, the beak is flattened from above downwards ; in others 

 it is curved upwards as in the avocet, or downwards as in the 

 curlew ; while the most remarkable beak of all is that of the tern, 

 known as the " scissor-bill." Herein the lower jaw is longer 

 than the upper, while both are laterally compressed to the thin- 

 ness of a knife-blade. This most curious bird feeds on fish, 

 and procures its food by swimming along the surface of the water 

 with the tip of the lower jaw submerged. As a shoal of fishes 

 is passed over, one often is caught up by this ploughshare, and 

 driven within the grasp of the upper jaw, where it is quickly 

 seized and passed backwards into the mouth. 



