XIII 



PHYLUM CHORDATA 



423 



having a uniform covering of feathers. The Ratitae, also, have 



nothing more than the merest trace of booklets on the barbules, so 



that the barbs do not interlock, and the vanes 



of the feathers are downy or hair-like. In 



the Penguins the wing-feathers are degenerate 



and scale-like. 



Many Birds are quite naked when hatched, 



but in most cases the body is more or less 



completely covered by a temporary crop of 



feathers, the nestling-downs, of various forms, 



but always having a short axis, soft, loose 



barbs, devoid of interlocking apparatus, and, 



except in the Emu, having no after-shaft 



(vide infra). They are succeeded, as already 



described, by the permanent feathers. 



Many Birds, such as the Swan, possess 



down-feathers or plumulce throughout life, in- 

 terspersed among and hidden by the contour 



feathers or pennce. In the Heron and some 



other Carinatae are found powder-down patches 



(Fig. 1060, B, p. d. p, p. d. p'), areas of downs, 

 the ends of which break off and make a fine 

 dust. Semi-plumes are downs with a well- 

 developed axis: filoplumes, as we have seen 

 (Fig. 1023, B), have an elongated axis and 

 vestigial vexillum. 



In many Birds there springs from the under 

 side of the quill, near the superior umbilicus, 

 a second vane, the after-shaft (Fig. 1061), 

 usually smaller than the main shaft, but some- 

 times of equal size. Both among Carinatse 

 and Ratitse we find genera with double- 

 shafted feathers and allied forms in which the 

 after-shaft is rudimentary or absent. 



The feathers are always shed or " moulted " 

 at regular intervals, as a rule annually. The 

 old feathers drop out and new ones are formed 

 from the same pulps. 



The colours of feathers present great variety. 

 Black, brown, red, orange, and yellow colours 

 are due to the presence of definite pigments, 

 i.e., are absorption-colours. White, and in 

 some cases yellow, is produced by the total 

 reflection of light from the spongy, air-contain- 

 ing substance of the feather, there being, as in nearly all other 

 natural objects, no such thing as a white pigment. Blue, violet, 

 and in some cases green, are produced by the light from a brown 



D D 2 



FIG. 1061. Feather of 

 Casuarius (Casso- 

 wary), showing after- 

 shaft and disconnected 

 barbs. (From Headley.) 



