The Snakelocks Anemone, Anemonia viridis, may be found in pools onshore and also in shallow water. It can be fairly large and has long, very mobile tentacles which can be retracted but rarely are. There are two colour forms - one, as here, in brown and another with green tentacles which have purple tips.
Found on southern and western shores only.
Muck, Small Isles, Western Scotland