Earth's atmosphere 3.5 billion years ago contained little or no oxygen.
Flatworms absorb oxygen by diffusion mechanism through their
skin to the cells in their bodies. Since flatworms do not have a circulatory system,
their bodies' cells are positioned close to the skin to ease the uptake of
oxygen from surrounding fluids or water.
Answer: it's d: has a bar through the middle from which arms extend....
Explanation:
It's on Quizlet
