506 



Comparative Morphology of Chordatet- 



adjacent barb. Each distal barbule (Fig. 396) has, along its deeper 

 edge, a series of minute sharply curved hooks. The outer ("outer" and 

 "deeper" in relation to the surface of the body) edge of a proximal 

 barbule is sharply curled over. Each hook on a distal barbule is firmly 

 caught underneath the curling flange of one of the several proximal 

 barbules which are overlapped by the distal barbule. Thus the succes- 

 sive sets of barbules are so firmly tied together that the whole system of 

 barbs and barbules is converted into a fairly rigid web or vane (vexil- 

 lum) capable of offering effective resistance to the air at the down- 



Fig. 396. Two enlarged barbs of a 

 feather, showing the interlocking bar- 

 bules. (Courtesy. Knowlton and Ridg- 

 way: "Birds of the World," New York, 

 Henry Holt & Co., Inc.) 



stroke of the wing. In the absence of this interlocking arrangement of 

 barbules, the feather is of a loose or fluffy texture, as is conspicuously 

 seen in ostrich plumes. Quite commonly an aftershaft, consisting of a 

 smaller secondary rachis equipped with barbs and barbules, projects 

 from the proximal end of the main rachis (Fig. 395). In the ostrich-like 

 birds, the emu (Dromiceius) and the cassowary (Casuariiis) , the after- 

 shaft is as large as the main feather so that, in effect, two equal feathers 

 spring from one quill. 



The quill is inserted into an appropriately deep pocket, the fol- 

 licle, extending obliquely into the skin (Fig. 397). Around the feather 

 the epidermis turns inward to form the wall of the follicle and, at the 

 bottom of the follicle, is continuous with the proximal end of the quill, 

 which is of epidermal origin. At least in the case of the larger feathers, 

 delicate strands of muscle-fibers connected with the deeper region of 

 the follicle extend obliquely upward through the dermis. By contrac- 

 tions of the muscle on a particular side of the follicle, the slope of the 



