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An excellent high quality picture Andy.
Some Description notes would help me - how deep you were and size of this one - anything really.
Underneath a rock at the extreme low water mark on a Spring Tide. It was about 2cm long when extended.
I've noticed they quite often are lacking some of their cerata like this one
Lat/Lng: 51.56654, -3.97809
OS grid ref: SS629871