Resting their heads
Japanese vocabulary term for today: tako no makura, or "octopus' pillow."
Googling tells me that it is Clypeaster japonicus, a kind of sea urchin and a close relative of the sand dollar. More pictures. Introduction in Japanese. And another. Punny reference in the Marioverse.
I learnt of these charmingly-named creatures via Hitomi Hitsudai 人見必大's 1695 Honchō shokkan (本朝食鑑, literally "A mirror [held up] to the diet of this country"), which, with typical RoboCop-like humorlessness, notes "So called because octopodes are said to use it as a pillow when they sleep. This is probably a joke invented by some woman or child." He also gives another name for the beast: the mochi kai 餅貝, kai meaning "shell[fish]."
Derek:
'Octopodes' is an awesome plural for octopus. Is this an alternative to 'octopi' that fell out of favor?