Answer:
exoskeleton
Explanation:
An exoskeleton is the hard outer covering that supports and protects some animals without backbones.
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1. Cell differentiation
2. Cell differentiation
3. Zygote
4. Programmed cell death
5. The cells in between were programmed to die
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The cell membrane is a lipid bilayer that prevents that passage of water and ions. This allows cells to maintain a higher concentration of sodium ions out the outside of the cell.
