CHAPTER XIX* 



THE SKIN AND SKELETON 



The Skin, or Integument, is that layer of tissue which 

 covers the outer surface of the body. The term Skele- 

 ton 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 muscles. 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. 



i. The Skin. In the lowest forms of life, as amoeba, 

 there is no skin. The protoplasm of which they are 

 composed is firmer outside than inside, but no mem- 

 brane is present. In Infusoria, there is a very thin 

 "cuticle" covering the animal (Fig. 9). They have 

 thus a definite form, while the amcebas continually 

 change. Sponges and hydras also have no true skin. 

 But in polyps, the outside layer of the animal is sepa- 

 rated into two portions ectoderm and endoderm 125 

 which may be regarded as partly equivalent to epider- 

 mis and dermis in the higher animals. These two layers 

 are, then, generally present. The outer is cellular, the 

 latter fibrous, and may contain muscular fibers, blood 

 vessels, nerves, touch organs, and glands. It thus be- 

 comes very complicated in some animals. 



* See Appendix. 

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