THE SKIN AND SKELETON. 127 



CHAPTER XVL* 



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. 



