I chose the neck of the tortoise to be my mechanism

After the researches, i found out that it has the strongest muscles that make its neck go inside the shell in a "S" shape

This is what happens inside

I made the object with the same mechanism,

A box that represent the shell.

Wooden cubes " the vertebral column of the tortoise"
and elastic bands that represents the muscles

And while pulling the elastics bands from behind the box, the wooden shapes go inside in a S shape