Hardly less diversified in form are the feet of birds. The skeleton of this part of the body is dealt with on another page; here we are concerned only with the external form of the feet and legs. Aquatic birds often have webbed feet, but not always. The Dipper, for example, is a bird which lives largely on and under the water, but its feet are not in the least like those of a Duck or Grebe. The webbed foot presents us with at least two varieties. In the Pelican tribe (fig. 5) the extreme of web-footedness is to be seen. Here all the toes (four) are connected by a webbing. In the Duck only three of the toes are webbed. Another kind of webbed foot is termed palmate. In the Coots, for example, each toe is fringed with a broad membrane, but there is no connection between the fringes of successive toes. The toes of birds are apt to be differently disposed. In most birds (fig. 6) there are three toes which are turned forwards, and one, the great toe (hallux), which is turned backwards. But in the Trogons and others two toes are turned forwards and two backwards, thus producing a very efficient mechanism for holding on tightly to the bough of a tree, a mechanism which is shared by that, in some other respects, bird-like lizard, the chameleon. A foot of this kind is technically called ‘zygodactyle.’ A singular modification of the foot is seen in the Kingfisher (fig. 7) where the two middle toes are enclosed in the same fold of skin; this is called ‘syngenesious.’ Skeleton. A bird’s skeleton is wonderfully light and spongy in texture. It is full of air (see below, p. 27), but deficient in marrow. Its entire structure is pre-eminently suited to a flying creature, not only for the above reasons, but because the heaviest part (the sternum) lies in the middle, in the centre of gravity, and thus assists in preserving the balance, like Blondin’s pole. The Skull. The skull of a bird is composed of a large number of separate bones, which are very closely united in the adult bird, so much so that it is next to impossible to recognise that they are distinct bones. The bones are also thin and light, for to a flying animal any weight forward would be most disadvantageous. The weight of the bird should be, and is, concentrated in the middle of the body. We can divide the skull into two regions: behind is the smooth, rounded brain-case or cranium; in front is the face, which is largely ensheathed by the beak. It is chiefly formed by the maxillary and nasal bones above, and by the palatine and pterygoids below. The length of this part of the skull is subject to great variation in different birds. In the Storks, for instance, the face is extremely long, while in the Parrots it is comparatively short.