Among _Swimming Ducks_ damage is only done by the =Wild Duck= (_Anas boschas_). It breeds wherever there are fresh waters. Nests amongst grasses or swamp-plants, or in a tree. The wild ducks remain the whole winter as long as the waters are not frozen, otherwise they go off for a short time to the south. Food: the tops of stems, buds, leaves of various water-plants, also barley, oats, and other grain; water-insects, fish, and fish-spawn. These ducks also do damage in cornfields by treading down and cracking the plants. Family: =Longipennes= (_Gulls_). Usually swimming feet. Legs tolerably long, adapted for wading (Fig. 45). Wings long, pointed. Beak laterally compressed. Young nestlings (p. 51). Breed in larger or smaller flocks, usually on the coast, occasionally on the margin of fresh waters. They chiefly feed on fish, worms, molluscs, and crustacea; sometimes, in the case of a few species, on young birds and eggs, as well as mice and other small mammals. Gulls are usually of no importance in agriculture; but the =Black-headed Gull= (_Larus ridibundus_), which breeds on the banks of lakes and rivers, devours many cockchafers and other noxious insects. The =Herring Gull= (Fig. 45), =Kittiwake= (_Larus argentatus_ and _L. tridactylus_), and a few other species, which breed on the coast, sometimes show themselves inland, especially in stormy weather; they then pursue field-voles, and catch many injurious insects. Besides the true =Gulls= (_Larus_), I will only mention the =Sea Swallows= (_Sterna_), with forked tails. [Illustration: FIG. 45.—Herring Gull (_Larus argentatus_). ] CLASS III.: =REPTILIA= (REPTILES). Cold-blooded Vertebrates (p. 19). The heart has the same structure as in Mammals and Birds, but the left and right ventricles are usually only incompletely divided from one another (p. 20). The body is invested with horny scales or with bony plates covered by a horny layer. Reptiles are very unlike Birds in appearance, but present many essential points of resemblance to them in their skeleton; indeed, during earlier geological times, there were transitional forms between the two classes. The structure of the reptile egg, too, is very similar to that of the bird’s egg; but the first has no lime-salts deposited in its shell. The egg is not hatched out by the animal itself, but exposed to the heat of the sun or to the warmth developed by decaying vegetable matter. Several reptiles (adder, for example) keep their eggs in their bodies till the young escape. Reptiles have either no limbs (snakes, a few lizards), or, at any rate, the limbs are not well developed, and are so placed that the body does not rest upon them, but is slung between them. [Illustration: FIG. 46.—Common Lizard (_Lacerta agilis_); 2, head of the same from below; 3, tongue. ]