PHYLUM CHORDATA 



381 



Exoskeleton. — The exoskeleton is purely epidermal, like that 

 of the Lizard, which it also resembles in consisting partly of horny 

 scales. These cover the tarso-metatai'sus and the digits of the foot, 

 and are quite reptilian in appearance and structure. Each digit 

 of the foot is terminated by a claw, which is also a horny product 

 of the epidermis ; and the healcs are of the same nature. The rest 

 of the body, however, is covered by feathers, a unique type of 

 epidermal product found nowhere outside the present class. 



A feather (Fig. 1023) is an elongated structure consisting of a 

 hollow stalk, the calamtos or quill (cal.), and an expanded distal 



rcJi 



Fig. 1023.— Columba livia. A, proximal portion of a remex. cal, calamus ; inf. umb. inferior 

 umbilicus ; rch. rachis ; jnyj. ?(m6. superior umbilicus. B, filoplurae. C, nestling-down. (C, 

 from Bronn's Thien'eich.) 



portion, the vexillum or vane. At the proximal end of the quill is 

 a small aperture, the inferior umbilicus (inf. umb.), into which fits, 

 in the entire Bird, a small conical prolongation of the skin, the 

 feather papilla. A second, extremely minute aperture, the superior 

 umbilicus {iup. iimh.), occurs at the junction of the quill with the 

 vane on the inner or ventral face of the feather, i.e., the face 

 adjacent to the body. A small tuft of down in the neighbourhood 

 of the superior umbilicus represents the after-shaft of many Birds — 

 including some Pigeons (vide infra). 



The vane has a longitudinal axis or rachis (rch.) continuous 

 proximally with the quill, but differing from the latter in being 



