The exoskeleton of the Duck and indeed of all birds is entirely epidermal in origin. Its most important part consists of =feathers=, but it includes also the following horny structures:-- (_a_) =scales=, which cover the toes and tarso-metatarsus; (_b_) =claws=, which are attached to the distal phalanges of the toes and of the pollex; (_c_) the wide =beak=, which sheaths both upper and lower jaws, and whose edges are raised into lamellae, which act as strainers. FEATHERS. A well developed feather, such as one of the large quill feathers of the wing or tail, consists of the following parts: A main stem, the =scapus=, which forms the axis running along the whole length of the feather, and is divided into (1) a proximal hollow cylindrical portion, the =calamus= or =quill=, and (2) a distal solid portion, the =rachis= or =shaft=, which is square in section, flexible and grooved along its ventral surface, and bears a number of lateral processes, the =barbs=. The =calamus= which is partly imbedded in a pit in the dermis, bears two holes: one, the =inferior umbilicus=, is at its proximal end, and into it enters a vascular outgrowth from the dermis; the other, the =superior umbilicus=, lies on the ventral surface at the junction of the calamus and scapus. The =barbs= are a series of narrow elastic plates, attached by their bases to the rachis, and with their edges looking upwards and downwards. The barbs are connected together by a number of smaller processes, the =barbules=, which interlock with one another by means of hooklets, and bear the same relation to the barbs that the barbs do to the rachis. The barbs and barbules, together with the rachis, constitute the =vexillum= or =vane= of the feather. Any feather having the above type of structure is called a =penna= or a =contour feather=, from the fact that it helps to produce the contour of the body. VARIETIES OF FEATHERS. 1. =Pennae.= There are two kinds of pennae or contour feathers. (_a_) The =quills=. These form the large feathers of the wing and tail. They are divided into two groups, the =remiges=, or wing quills, and the =rectrices=, or tail quills. The =remiges=[101] include three sets of feathers, the =primaries= or =metacarpo-digitals=, which are attached to the bones of the manus, the =secondaries= or =cubitals=, which are attached to the ulna, and the =humerals=, which are attached to the humerus. The =primaries= differ from all the other quill feathers in having the posterior half of the vane much wider than the anterior half. They are ten in number, and of these six, the =metacarpal= quills (fig. 57, 14), are attached to the second and third metacarpals, one, the =ad-digital= (fig. 57, 15), to the phalanx of the third digit, two, the =mid-digitals= (fig. 57, 16), to the first phalanx of the second digit, and two, the =pre-digitals= (fig. 51, 17), to the second phalanx of the second digit. One of the pre-digitals is very small, and is called the =remicle= (fig. 57, 11).