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Lampyris noctiluca
I am guessing this is some type of Lampyridae (beetle) larva
Google Lampyridae larva and look at images - you will find some similar looking things.
Nice find! I've moved this to the Arthropod category for you. Arthropod = joints, segments, hard cuticle that is moulted (caterpillar, crab, spiders, etc).