I COLOUR 
Os 
the barrel or quill (calamus or seapus) at the base of the tuft 
of rays have elongated into a principal shaft (rhachis) ; this is 
generally accompanied by a secondary “ aftershatt ° (hyporhachis), 
originating from the “ ventral” side, which in the Emeu and 
Cassowary rivals the shaft itself in size. On the rhachis a double 
series of lamellae or barbs are developed, carrying a similar double 
series of barbules, much as in the down-feather, but the barbules 
again give rise to barbicels (cia), which in the distal rows 
usually terminate in hooklets (damuli). These catch in the folded 
margins of the next proximal row, and a firm surface is thus 
secured, An after-shaft never, and a down-feather rarely, possesses 
barbicels; while in some cases by the absence of these and part 
of the barbules a “ disconnected ” web and a “ decomposed ” feather 
are formed, as in the decorative tufts of many species. The barbs 
may even be absent, as in the wing-quills of Cassowaries, the 
“wires” of Birds-of-Paradise, the “ bristle-feathers” at the gape 
of Night-jars or the eyelashes of Hornbills. In the hackles of 
Gallus (Fowl), and the secondaries or even the tail-feathers of 
Ampelis (Waxwing), the tip of the rhachis is flattened and wax- 
like; and similar structures are observable elsewhere. In the 
newly-hatched young the down is often partly or entirely sup- 
pressed, but in certain Birds this suppression is temporary, and a 
thick coat grows after a few days. “ Powder-down” feathers are 
those which never develop beyond the early stage, and continually 
cusintegrate at the tip into bluish- or greyish-white powder ; they 
occur in the Zinamidae, Ardeidae, Rhinochetidae, EBurypygidae, 
Mesitidae, Accipitres and Psittaci, in Podargus, Coracias, Lepto- 
soma, Gymnoderus and Artamus. 
Colour.—The colour of Feathers is due to one of three causes. 
First, an actual pigment ' may be present in certain corpuscles, or 
in diffused solution, and the tint does not then vary according to 
the incidence of the hght. Secondly, it may arise from a pigment 
overlaid by colourless structures in the form of ridges or imbedded 
polygonal bodies; here, if the vanes are scraped or held up to 
the light, the pigmentary colour alone is visible? Thirdly, the 
colour may be iridescent or prismatic; that is, a blackish pig- 
1 Of this nature are zoomelanin (black), zoonerythrin (red), zooxanthin (yellow), 
turacin (red—only known in the Musophagidae), and perhaps turacoverdin (green, 
from the same family). Brown is produced by a combination of red and black ; 
white is the appearance due to innumerable air-spaces. 
* Such are many yellows, oranges, greens and blues. 
