THE SKIN AND SKELETON. 135 
“crust.” The latter is an outer layer of minute over- 
lapping scales, which are directed 
toward the point, so that rubbing YY 
a human hair or fibre of wool be- AE 
tween the thumb and finger pushes MY 
the root-end away. The root is 
bulbous, and is contained in a mi- 
nute depression, or sac, formed by 
an infolding of the skin. Hairs are 
usually set obliquely into the skin. 
Porcupine’s quills and Hedgehog’s | 
spines make an easy transition to Q\\/\\\¥é 
feathers, which differ from hairs «\ 
only in splitting up into numerous 
lamine. They are the most com- 
plicated of all the modifications of 
the epidermis. They consist of a 
“quill” (answering to the bulb of 
a hair), and a “shaft,” supporting 
the “vane,” which is made up of 
“barbs,” “ barbules,” and interlock- 
ing “processes.” The quill alone 
is hollow, and has an orifice at 
each end. The teeth of Mollusks 
and Articulates are also epidermal *"? Prerepee i 
structures; but the teeth of Verte- ane 0" Deard; 4, accessory 
plume, or down; e, f, lower 
brates are developed from the der- —2"4 upper umbilicus, or ori- 
fice, leading to the interior of 
mis. In all cases, teeth belong to _ the quill. 
the exoskeleton. A human tooth and an oyster-shell rep- 
resent each other, structure for structure. 
(2) The Endoskeleton, as we have seen, has its first rep- 
resentative in the Cuttle-fish. With this exception, it is 
peculiar to Vertebrates. In the Outtle-fish, and some 
Fishes, as the Sturgeon and Shark, it consists of carti- 
lage; but in all others (when adult) it is bone or osseous 
