CHAPTER 6 

 THE SKELETAL SYSTEM 



Some creatures, jelly fish for example, have no skeleton. In some, 

 as in many molluscs and more conspicuously the corals, the skeleton is 

 heavier than all the soft parts combined. In man, the bones make up 

 about a fifth of the entire weight of the body, and this is not far from 

 the average for active air-breathing vertebrates that do not have dermal 

 armor. 



All skeletons support or protect the softer parts. Supporting skeletons 

 occur even in such lowly creatures as protozoans and sponges. Protective 

 skeletons are conspicuous in echinoderms and molluscs, are universal 

 among the arthropods, and are found among vertebrates in such diverse 

 groups as the Paleozoic ostracoderms, ancient and modern ganoids, 

 dinosaurs, turtles, and armadillos. 



Skeletal parts, which are also jointed levers used in locomotion, occur 

 in arthropods and vertebrates alone. 



Arthropods solve the problem of locomotion by means of a chitinous 

 exoskeleton with the muscles inside it. Such a skeleton is highly efficient 

 as attachment for muscles, and it has the further advantage of providing 

 armor at the same time. Its disadvantage is that it cannot grow, so that 

 all the arthropods, by one device or another, shed their exoskeletons as 

 their bodies enlarge. This leaves them for a time helpless. Furthermore, 

 since the tissues of the molting arthropod are unsupported, no arthropod 

 can attain any considerable size. Among arthropods the largest air- 

 dwellers are foot-long centipedes; and although among water-dwellers 

 the euripterids of the lower Paleozoic and earlier were more than a yard 

 long, a twenty-pound lobster is about the limit for a modern form. The 

 typical arthropod is a tiny insect. 



The endoskeleton of vertebrates, light and strong, and capable of 

 indefinite growth, has the single disadvantage that skeletal armor must 

 be developed independently. But vertebrates have for the most part 

 abandoned armor. Their success as a group has depended not a little 

 on their admirable endoskeleton. To its usual functions, the vertebrates 

 add the production of blood cells by the marrow, especially in the long 

 bones. 



The Two Parts of the Skeleton. Historically, the vertebrate skeleton 

 consists of two parts, which began independently, have evolved separately, 



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