Ever wondered what would happen if a jellyfish mated with a sea star? I haven't. But if I had, this is probably what I would have come up with: a large, gelatinous star. This echinoderm, is known as the hymenaster sea star, or giant slime star, and it gets to be about a foot across.
Really, if you're going to do the whole jelly thing, why not be giant too? Then again, given my lack of true biology training, this could be an ulcerous stomach recently removed from a corpse for all I know.