136 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



that there were successive ages when skeletons were enormously large and 

 heavy. Those of reptiles commonly weighed many tons. Even modern alli- 

 gators have such heavy ones that they can scarcely lift their bodies from the 

 ground. 



During their evolution vertebrate skeletons have changed from ponderous 

 burdens to light jointed bones, adapted to muscular control. Of all the land 

 vertebrates, birds have the lightest skeletons, for their tubular bones contain 

 air cavities connected with the lungs. The frigate bird, a famous flier, has a 

 wing expanse of seven feet and weighs two pounds, but its skeleton weighs 

 only four ounces, less than its feathers. 



Types of Skeletons 



Skeletons are either exoskeletons, on the outside of the body, or endoskele- 

 tons, within the body. 



Exoskeletons of invertebrates are composed entirely of nonliving material, 

 the secretion of cells usually deposited in layers (Fig. 9.2). The majority are 

 light in weight, except the shells of mollusks that are often heavy. The muscles 

 are attached on the inner surfaces of the shells (Fig. 9.5, crayfish). 



Endoskeletons are composed of living cells with their products, such as the 

 limy substance of bone. They are located between muscles and connective 

 tissues, and the muscles are attached to their outer surfaces. Such skeletons 

 are unique to the great group of chordates presently described. 



Skeletons of Invertebrates 



In the vast assemblage of invertebrates there is an unending variety of 

 skeletons that fit their owners to live in thousands of niches, in water, on land, 



Fig. 9.1. Skeletons of representative radiolarians of crystal transparency, beauty 

 and precision of pattern. A vast area of the ocean bottom is covered with ooze 

 mainly composed of these skeletons that have dropped downward and accumulated 

 through the ages. (Courtesy, Kudo: Protozoology, ed. 3. Springfield, 111., C. C 

 Thomas, 1947.) 



