XIJ1 



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-metatarsus and the digits of the foot, 

 and are quite reptilian in appearance and structure. Each digit 

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

 of the epidermis ; and the beaks 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 calamus or quill (cal.), and an expanded distal 



7V7/1 



Fin. 1023. Columba livia. A, proximal portion of a reraex. cal. calamus; inf. umb. inferior 

 umbilicus ; rch. rachis ; sup. umb. superior umbilicus. B, filoplmne. C, nestling-dowu. (C, 

 from Brown's Thierreich.) 



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 (sup. icmb.), 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 



