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 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 mantle. 



