Page:Natural History, Birds.djvu/301

288 the foot in the water an impetus, which is very advantageous in swimming. The tarsus is commonly flattened sidewise, that less resistance may be offered to progression in so dense a medium. "In order to make the stroke, the foot is first drawn forwards, when the toes close together and the webs fold, so as to offer to the water the least possible resistance; but when the back stroke is made, the toes spread out, while the action of the limb is at the same time, in most instances, obliquely outwards."

The form of the body is flattened, not however laterally as in the Waders, but horizontally, the better to float on the surface; the breast-bone is very long, affording a bony protection to the greater portion of the intestines. The plumage is remarkably thick and close, particularly on the under parts of the most aquatic kinds; besides which the skin is furnished with a dense coat of soft down. The outer surface of the plumage is in general polished and satiny, having the property (perhaps from being anointed with an oily secretion frequently applied by the beak) of throwing