XIIl 



IMIYLUM CHORDATA 



;wi 



Exoskeleton. — The cxoskelcton is purely cpidennal, like tliat 

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

 scales. These cover the tarso-metatarsus and the (ligits of the foot, 

 and are quite reptilian in appearance and structure. Each digit 

 of the foot is terminated by iirhdr, which is also a hoi-ny pi-oduct 

 of the epidermis; and the J>c(il'.^ are of the same nature. The rest 

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

 epidermal |)roduct found nowhere outside the present class. 



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

 hollow stalk, the calamus or quill {c((l.), and an expanded distal 



rt^/i 



Fig. 1023.— Columba livia. A, prnxinial iiortion of a remex. caL calamus ; inf.uwh.interiov 

 urnbilkus ; ,v/t. laclii.s ; sm^j. »/(*/<. superior umbilicus. B, filoplunie. C, nesUing-down. (C, 

 fnim Bronn's Thierreich.) 



portion, the rccUlnm 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 ixqnlla. A second, extremely minute aperture, the superior 

 umbilicus (su.p. umb.), occurs at the juncfc-ion 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 {cide infra). 



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

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



