The _integument_ differs markedly from that of other animals in being
clad with feathers. Three distinct kinds of feathers are at once
distinguishable--(_a_) the small hair-like downy rudimentary
_filoplumes_; (_b_) the numerous smaller contour or covering _plumes_;
and (_c_) the large strong quill-feathers or _pennæ_ on wings and tail.
The ordinary feather consists of a quill at the base of a shaft up the
center, and of the vane borne on the sides of the shaft. The vane
consists of parallel barbs, which are linked together by small barbules.
On the bare legs of many birds the feathers are replaced by horny
scales, and the horny structures forming the beak and terminating the
toes are very familiar.