9 



Protection, Support, and 

 Movement — Skeletons 



Skeletons provide protection and support. The advantage of having a skele- 

 ton is made most vivid by the animal which does not have one. Jellyfishes drift 

 and in calm seas can even swim. But let them be thrown on a sandy beach and, 

 having neither protection nor support, they flatten against the sand and dry to 

 papery wisps. All vertebrate animals have skeletons and the character of their 

 existence is inseparable from skeletons. Imagine a spirited horse without bones! 

 In their relations to their environments and their achievements of speed, 

 strength, and grace animals are greatly dependent upon an outer or an inner 

 frame. 



General Functions 



The skeleton determines the form of an animal. Contrast the long leg bones 

 of an ostrich and the lack of them in a snake; or the seven long vertebrae in 

 the neck of a girafi'e and the seven short ones in the neck of a man. 



Bones are the living tools of the muscles. Watch the fingers striking piano 

 keys, or the legs taking part in defense when a donkey kicks, and in offense 

 when a cat springs upon a mouse. 



The skeleton's oldest and most general function is protection. The shell is 

 a complete armor around a lobster; the boxlike cranium encases the human 

 brain. The red marrow that produces the vital blood cells of vertebrates 

 throughout adult life is housed within bones. 



Skeletons are old in animal history. Even in early times the yielding proto- 

 plasm of the smallest animals was doubtless protected by shells and rodlets of 

 hardened secretion as radiolarians are now (Fig. 9.1). Tons of fossil deposits 

 that have been dredged from the sea bottoms testify to the abundance of such 

 microscopic skeletons in primeval seas. Fossil animals of other groups show 



135 



