THE SKIN AND SKELETON. 
137 
feathers, which differ from iiairs only in splitting up into 
numerous laminae. They are the most complicated of all 
the modifications of the epidermis. 
They consist of a “quill” (answer¬ 
ing to the bulb of a hair), and a 
“shaft,” supporting the “vane,” 
which is made up of “ barbs,” “ bar- 
bules,” and interlocking “process¬ 
es.” The quill alone is hollow, and 
has an orifice at each end. The 
feather is moulded on a papilla, the 
shaft lying in a groove on one side 
of it, and the vane wrapped around 
it. When the feather emerges from 
the skin, it unfolds itself. Thus 
shaft and vanes together resemble 
the quill split down one side and 
spread out. 
The teeth of Mollusks, Worms, 
and Arthropods are also epidermal 
structures. Those of Vertebrates are 
mixed in their origin, the dentine be¬ 
ing derived from the dermis and the 
enamel from the epidermis. In all 
cases teeth belong to the exoskeleton, 
(2) The Endoskeleton, as we have 
seen, is represented in the Cuttle- Vane, or beard; d, accessory 
: plume, or down; e, /, lower 
fish. With this and some other and upper umbilicus, or ori- 
.... .. , TT fice, leading to the interior 
exceptions, it is peculiar to Verte- of the quin, 
brates. In the Cuttle-fish, and some Fishes, as- the Stur¬ 
geon and Shark, it consists of cartilage; but in all others 
(when adult) it is bone or osseous tissue. Yet there is a 
diversity in the composition of bony skeletons; that of 
fresh-water Fishes contains the least earthy matter, and 
that of Birds the most. Hence the density and ivory- 
