THE SKIN AND SKELETON. 
127 
CHAPTER XVI * 
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 protoplasm of which they are com¬ 
posed is firmer outside than inside, but no membrane is 
present. In Infusoria, there is a very thin cuticle cover¬ 
ing the animal. 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 cel¬ 
lular, 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 mantle, 
* See Appendix. 
