I have been finding these for years at the beach, never knowing what they were. They cling to seaweed, sometimes in the hundreds, and so I'd take a clump and put it in a bucket and watch them all disperse into the water. They seem to come in a variety of colors and sizes, most too small for me to photograph (though I have photos of a few others, red ones). This is the largest one I've found. I'm really interested to know what these are, they're a part of my childhood!