Land planaria, sometimes known as arrow-head worms or hammer-head worms, of the genus Bipalium.
Its Japanese name is kogaibiru (笄蛭), named after an ornamental hair pin.
This is a predatory flatworm. It feeds on earthworms, snails and slugs. Some species are invasive – Bipalium nobile, which can reach up to 1 metre in length, for example.
Primarily nocturnal, and living underground or under leaf litter, this is a creature that is frequently close to human habitation but generally escapes notice – few Japanese people I’ve talked to know of its existence.