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Type: Multiple-Choice
Category: Zoology
Level: College
Created: 2 weeks ago

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College Zoology

Why do sponges, cnidarians, and flatworms lack a specialized gas-exchange surface?
  1. Nearly all of their cells are in direct contact with the external environment.
  2. Countercurrent exchange mechanisms cannot function well in their living conditions.
  3. They do not produce carbon dioxide.
  4. They are too large for a circulatory system to operate well.
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