The feathers may be considered of two chief kinds, the quill feathers and the clothing feathers. The most rudimentary of the latter are known as down. A _quill feather_ consists of two principal parts: the quill, or calamus, and the vane, or vexillum. The quill is continuous with the central shaft, called the rachis, the two forming the stem of the feather. Projecting outward from the stem on each side are a large number of pointed and very flexible _barbs_. These barbs are located nearly at right angles to the quill and have extending from them at right angles smaller processes or _barbules_. These barbules hook together the barbs and give the web its form. In some feathers, as the hackle, and in the wing bar feathers of the male of some breeds, there is a portion of the upper outer edge of the feather not provided with barbules, which fact gives the feather its characteristic appearance. The down feathers are loose and fluffy. In this kind of feather the shaft is weak, and the barbs are not provided with barbules. The barbs like the shaft may be considered weak. These feathers give great warmth to the body. There are fiber feathers appearing as hair-like filaments, and called _filo-plumæ_. These are found scattered over the body; they are particularly abundant in the region of the head and the neck. At each end of the quill is a small opening, or _umbilicus_. Inside the barrel of the feather there is _pulp_, which in young feathers is very vascular; the vessels entering by the proximal umbilicus are buried, along with part of the quill, in a papillated follicle of the skin. From this follicle the feather is developed. At the base of the shaft a secondary rudimentary quill is usually formed, which may be represented by a mere tuft of down. On the same general principle the smaller feathers are constructed. These cover the body, the upper parts of the legs, and the head, while the larger feathers and quills are confined to the wings and the tail. The longest quill feathers are those arising from the hand, called the _primaries_. Those arising from the forearm are called the _secondaries_. Those that are developed from the proximal part of the arm are called the _tertiaries_. The rudimentary pollux carries some feathers which form the _alula_, or bastard wing. The scapularies are feathers covering the scapula and the humerus. Covering the bases of the larger flight feathers are wing coverts consisting of several rows of small feathers. The quill feathers of the tail are called the _rectrices_. The rectrices have considerable mobility; their bases are covered by a row of tail coverts. The pedal digits of the natatores are joined by a membrane, covered with scaly skin, which forms the web foot.