Remember what creatures need in order to grow shells and hard structures. Calcium ionsCarbon dioxideHydrogen ionsCarbonate ions and calcium ions

Creatures need both carbonate ions and calcium ions to grow shells and hard structures. Here's a step-by-step explanation:

1. The process begins with the creature's body extracting calcium ions and carbonate ions from its surrounding environment. This could be from the water (in the case of aquatic creatures) or from the food it eats.

2. These ions are then transported to the specific body part where the shell or hard structure is being formed.

3. Once at the site, the calcium ions and carbonate ions combine to form calcium carbonate, which is the main component of shells and other hard structures in many creatures.

4. The calcium carbonate is then deposited layer by layer, gradually building up the shell or hard structure over time.

5. This process continues throughout the creature's life, allowing the shell or hard structure to grow and repair itself as needed.

So, in summary, creatures need both carbonate ions and calcium ions to grow shells and hard structures.