306 THE VERTEBRATE SKELETON. 



(b) The tectrices or coverts are short feathers, which 

 cover over the quills of the rectrices and remiges, and clothe 

 the body generally. Their barbules are less developed than is 

 the case with the quill feathers, so that the barbs separate 

 readily from one another, especially at the base of the vane. 

 The nomenclature of the various patches of coverts on the 

 wings is seen in fig. 56. A small patch of backwardly-directed 

 feathers surrounding the external auditory opening are known 

 as the auriculars. 



2. The filophimes are rudimentary feathers, consisting 

 of a minute stem and slightly developed vane. They are left 

 in the skin after the other feathers have been removed. 



3. The phimulae, or down feathers, have the stem very 

 slightly developed, while the barbs are soft and free from one 

 another. They are distributed all over the body, not only 

 among the contour feathers, but also over the spaces (apteria) 

 which bear no contour feathers. 



In the young bird the rudiments of the new feathers are 

 formed at the bases of the embryonic down feathers, and as 

 they grow they push them out from the skin. The embryonic 

 down feathers however remain attached to the apices of the 

 new feathers till these have reached a length of about an 

 inch; they are then shed. 



II. ENDOSKELETON. 



As compared with that of the Turtle or Crocodile, the 

 endoskeleton of the Duck is characterised by : 



1. The great lightness of the bones, many of which con- 

 tain air cavities. 



2. The tendency to become ankylosed together shown by 

 many of the bones. 



3. The modification of the anterior limbs and girdle for 

 the purpose of flight. 



