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What are homologous structures? Give an example. Is it necessary that homologous structures always have a common ancestor?
 
a. __________ have distinct roles and appearances, yet they all have the same basic anatomical plan and origin.
 
b. ________ refers to the development of homologous organs by animals evolving in separate directions from a common ancestor.
 
c. Is it necessary that homologous structures always have a common ancestor?