Diario de floundark

Okay, so around last year, I found a Polychaete (I think). It was dull out of water and vibrant in water. it had two giant brown spots on one end. it looked like it had poisonous markings, I picked it up, and never had any reaction, so I don't think it was poisonous. It was very odd. I am still confused as to what this is. it almost looks like a long anemone. it rarely moved. does anybody know what this is?