Here's why:

* Insects: Insects have exoskeletons made of chitin, a tough, flexible polysaccharide.

* Roundworms (Nematodes): Roundworms have a cuticle, which is a tough, flexible, non-living outer layer made of collagen.

Calcium carbonate is the primary component of shells in animals like mollusks (snails, clams), crustaceans (crabs, lobsters), and some types of corals.

Let me know if you have any other questions!