"It is known that many reptiles, and above all the snakes, cast off the
whole skin at once, whereas human beings do so by degrees. If by any
accident they are prevented doing so, they infallibly die, because the
old skin has grown so tough and hard that it hinders the increase in
volume which is inseparable from the growth of the animal. The casting
of the skin is induced by the formation on the surface of the inner
epidermis, of a layer of very fine and equally distributed hairs, which
evidently serve the purpose of mechanically raising the old skin by
their rigidity and position. These hairs then may be designated as
_casting hairs_. That they are destined and calculated for this end is
evident to me from the fact established by Dr. Braun, that the casting
of the shells of the river crayfish is induced in exactly the same
manner by the formation of a coating of hairs which mechanically loosens
the old skin or shell from the new. Now the researches of Braun and
Cartier have shown that these casting hairs--which serve the same
purpose in two groups of animals so far apart in the systematic
scale--after the casting, are partly transformed into the concentric
stripes, sharp spikes, ridges, or warts which ornament the outer edges
of the skin-scales of reptiles or the carapace of crabs.
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