The esophagus has a simple but significant functions to animals who has it. These functions can be a) to swallow food and serve as a passageway to the stomach and b) may serve to release the eaten food that might have some harmful compositions or chemicals -the vomit response. 
        
             
        
        
        
 
Instead of roots, threadlike structures called rhizoids. 
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Answer:
plants, particularly in terrestrial ecosystems, and algae in aquatic ecosystems. If the plants or other producers of an ecosystem were removed, there would be no way for energy to enter the food web, and the ecological community would collapse.