beach bug - possibly insect larva?

beach bug - possibly insect larva?

Observation - beach bug - possibly insect larva? - UK and Ireland. Description: Fast moving, black, segmented creature found in rocks/sand at edge of beach. Obvious head, but no obvious thorax/abdomen. Possibly missing legs but looks like having 3 pairs.Last segment has small feelers. Looks a little like a glowworm/ large ladybird la

Fast moving, black, segmented creature found in rocks/sand at edge of beach. Obvious head, but no obvious thorax/abdomen. Possibly missing legs but looks like having 3 pairs.Last segment has small feelers. Looks a little like a glowworm/ large ladybird larva