THE SKIN AND SKELETON. 
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CHAPTER XYI. 
THE SKIN AND SKELETON. 
The Skin, or Integument, is that layer of tissue which 
covers the outer surface of the body. The term Skeleton 
is applied to the hard parts of the body, whether external 
or internal, which serve as a framework or protection to 
the softer organs, and afford points of attachment to mus¬ 
cles. If external, as the crust of the Lobster, it is called 
Exoskeleton ; if internal, as the bones of Man, it is called 
Endoskeleton. The former is a modification of the skin; 
the latter, a hardening of the deeper tissues. 
1. The Skin.—In the lowest forms of life, as Amoeba, 
there is no skin. The jelly of which they are composed 
is firmer outside than inside, but no membrane is present. 
In Infusoria, there is a very thin cuticle covering the ani¬ 
mal. They have thus a definite form, while the Amoebae 
continually change. Sponges and Hydras also have no 
true skin. But in Polyps, the outside layer of the animal 
is separated into two portions—ecderon and enderon 71 — 
which may be regarded as partly equivalent to epidermis 
and dermis in the higher animals. These two layers are, 
then, generally present. The outer is cellular, the latter 
fibrous, and may contain muscular fibres, blood-vessels, 
nerves, touch-organs, and glands. It thus becomes very 
complicated in some animals. 
In Worms and Arthropods, the cellular layer, here 
called hypodermis, excretes a structureless cuticle, which 
may become very thick, as in the tail of the Horseshoe 
Crab, or may be hardened by deposition of lime-salts, as 
in many Crustacea. The loose skin, called the manile , 
