INTRODUCTION TO THE VERTEBRATES 



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coupled, in most cases, with an inactive life. This made for safety 

 first, but limited increase in activity and intelligence. 



The arthropods, especially the insect class, tried what could be 

 done with an external protective skeleton, but one provided with 

 joints, so that activity need not be sacrificed to safety. This has 

 produced the winners in life's race, if numbers be the standard. 



FIG. 82. Showing endo- and exo-skeletons. 



The bones in a man's leg are surrounded by muscles; the 

 skeleton of a grasshopper's leg consists of tubes with muscles 

 inside. From Pearse. 



But the external skeleton and the ventral nervous system imposed 

 obstacles to large increase in size, on the one hand, and to a highly 

 developed brain, on the other. 



A third line of development, with the internal skeleton and the 

 nervous system dorsal in the body, was attempted by the group 

 of animals called the vertebrates. This permitted great increase 

 in size both of body and brain, and while giving less protection, 



