which living animal sleeps on his back.

I have seen baby polar bears sleep on their back - just like a human baby and when they stir and shudder, it's as if you're watching a human but in bear form... I have also seen dogs lie on their back when they want their tummy stroked, and cats are always rolling around on their backs. I think it's generally the babies that normally sleep on their back but not sure why they end up sleeping in a 'sitting' postition.

i can't think of anything but spiders. you see spiders do not sleep on their backs but their body are over their backs.