Feathers have evolved to serve a variety of functions --- flight, heat conservation, waterproofing, camouflage, and display. Each feather consists of a tapering shaft bearing a flexible vane on either side. The exposed base of the shaft is called the calamus or quill. If viewed by cross section, the calamus is round and hollow. An opening at the bottom of the calamus, called the lower umbilicus, allows blood to enter the young feather during its short growth period. When its growth is completed, the feather is sealed off and, although it may be moved by a separate muscle situated in the skin, the feather itself is "dead." The stiff shaft running through the center of each feather is called a rachis, and the inner and outer vanes carried by the rachis are composed of a row of barbs, arranged side by side. Each barb in turn contains many tiny branches, set side by side, called barbules. Six types of feathers shown above are contour, semiflumes, filoplumes, down, powder down and bristle.