Brains on the beach

Tonight, Daddy, William, Charles, Austin, and I walked through LBTS, and then we walked home on the beach. There were a number of round, squishy mussel creatures that looked like little walnut sized brains. There were several stranded on the beach. I told the boys to look at the "brains" and they thought it was so cool. I passed a family whose son tried to pick one up. I asked if they knew what they were, and the mom said another man on the beach said they were "sea borgs." She was going to look it up. We have tried and cannot find it. The mystery creatures made an impression on William because he just told me that he doesn't want to go on the beach tomorrow because "those brains are smart. Every time Austin and I would stop to look at them, they would give us a headache. Those brains are smart."