Here's why:

* Structure and Function: Both the plant cell wall and the crab shell provide structural support and protection. The cell wall gives the plant cell its shape and protects it from bursting due to internal pressure. The crab shell protects the crab from predators and helps it maintain its shape.

* Material: The cell wall is mainly composed of cellulose, a strong and rigid material. The crab shell is made of chitin, a similar but slightly different complex carbohydrate.

* Rigidity: Both the cell wall and the crab shell are rigid and non-flexible. They provide a tough outer layer that maintains the organism's form.

The skeleton of a dog is made of bone, a very different material that is more flexible and dynamic than the cell wall or crab shell.