I read that these are under the same phylum as jellyfish, and are often mistaken for jellyfish. However, they are not. There were many of them washed up on the beach last weekend of many different sizes. This one was about 4 centimeters in diameter. I loved the color so I had to pick it up and study it a little bit! A few people were stung by it while I was at the beach and it did not look fun.