CHAPTER 6 



THE SKELETAL SYSTEM 



With but few exceptions, all animals have some type of supporting 

 structure. These structures vary from the simple strengthening rods 

 found among some single-celled animals to the complex inner skeleton 

 of the vertebrates. 



Many dififerent types of structures for support have been developed 

 within the various animal groups. Among the molluscs (the snails, 

 clams, octopuses, etc.) some forms have heavy outer shells, others such 

 as the octopus actually have an internal "cuttlebone." The starfish has 

 heavy calcareous plates ; corals secrete a limy coating. Among the in- 

 numerable animals without backbones, the invertebrates, the most suc- 

 cessful have been the insects and their allies. Their success as a group 

 is in a great part due to their having a skeleton on the outer surface 

 of the body. This is the exoskeleton which is formed of a heavy 

 chitinous material. By contrast the vertebrates, whose success as a 

 group is certainly comparable to that of the insects, have a hard in- 

 ternal skeleton, the endoskeleton, composed of bone or cartilage. 



FUNCTIONS OF A SKELETAL SYSTEM 



No matter what type of supporting structure an animal has, cer- 

 tain functions are performed by it. 



1. Support. For any animal, one of the chief functions of any 

 skeletal structure is to give support to the softer body parts. In the 

 various invertebrate groups, this function is frequently secondary to that 

 of protection. Among the vertebrates, however, support has assumed 

 a greater importance as other structures take over the function of pro- 

 tection. The large size of many of the vertebrates, both living and 

 fossil, has increased the need for more efficient support. 



2. Protection. The exoskeleton of the insects and the heavy shell 

 of the clam obviously are chiefly protective in their function. The soft 



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