CHAPTER 1 1 



ANIMALS WITH JOINTED FEET— 

 THE ARTHROPODS 



The body plan of the arthropods is suf- 

 ficiently plastic and flexible to permit mem- 

 bers of the phylum to fit into most of tlie 

 niches of the earth. Although the arthro- 

 pods resemble the annelids in many re- 

 spects, they show important innovations. 

 One of these is the hard outer skeleton, the 



that the muscles can be attached to the 

 inside of this material and function more ef- 

 ficiently in moving the body. The exoskele- 

 ton supports the entire animal, much like 

 the framework of an airplane or automobile. 

 One can well realize the difficulty of trying 

 to fasten the engine of an airplane to a soft 



exoskeleton, which functions as a rigid coat fuselage. For the same reason the plan of 

 of armor. It is as if the cuticle of the annelid the exoskeleton as an over-all superstruc- 

 had become thick and rigid. This means ture is apparently a very good one. It is 



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