XII AVES 429 



existing portion is pushed out of the follicle. As this happens it breaks 

 off and is shed. 



It follows that the successive " feathers " that develop from the 

 same follicle at successive moults are as a matter of fact not separate 

 individual feathers but merely successive portions of a single continuous 

 structure. Fig. 184, B, illustrates an interesting abnormality in 

 which two of these successive instalments of feather retain their con- 

 tinuity, the distal portion having failed to become detached at the 

 preceding moult. 



The first instalment of feather developed from each follicle differs 

 from its successors in being a down feather, a short quill bearing a crown 

 of barbs which are not arranged in one plane and are not coherent but 

 free and fluffy. The coating of down-feathers is succeeded by the 

 ordinary coats formed mainly of overlapping contour feathers like that 

 described. 



In particular regions of the body occur feathers more or less modified 

 in character. Along the edge of the wing are the specially long and 

 stiff flight feathers (remiges) divided into the primaries, attached to the 

 hand region, and the secondaries, attached to the forearm. In these the 

 posterior series of barbs are considerably longer than the anterior, so 

 that the air pressure causes them to bend upwards during the downward 

 beat of the wing so as to bring about a forward thrust to the body. 



Another set of large stiff feathers are the tail-feathers or rectrices, 

 most usually 12- in number, which play an important part in steering 

 the bird and in checking its flight when alighting. 



Types of feather more highly modified are the filoplumes, scattered 

 about amongst the ordinary feathers, in which the vexillum is greatly 

 reduced in size ; and the bristles, frequently found at the base of the 

 beak, or occupying the place of eyelashes, in which the vexillum has 

 disappeared entirely. 



Well-developed claws are present on the toes, but in the case of the 

 wing the degeneration of the digits has been accompanied by the dis- 

 appearance of claws. It is of evolutionary interest to note that the 

 ancient and relatively primitive bird Archaeopieryx possessed well- 

 developed claws on all three digits of its wing and that they turn up not 

 unfrequently (to the number of 2 or i) in various modern birds. In the 

 curious South American bird, Opisihocomus, the young bird is able to 

 clamber about quite actively by means of the two claws on its wing. 



As is the case in Reptiles so also in the Birds the glands of the skin — 

 so numerous in Fish and Amphibians — are greatly reduced in number. 

 In a typical bird in fact the only well-developed gland of the outer skin 



