Echidnocerus foraminatus (species)

The brown box crab is an uncommon crab living on the Pacific coast of North America, which forms an odd cylindrical opening when its limbs are pulled together. These holes are called "foramens", and are used to allow the crab to breathe while buried in sand on the sea floor. Like its sister species, it's also sometimes referred to as "Lopholithodes foraminatus", a synonym unaccepted by WoRMS.

Look closely at the right claw of the crab in the above image (photographed by who else but the Lithodoidea photographer celebrity, Neil McDaniel), and you can see one half of its circle-shaped hole. The other half is on the limb below it.