These are panda skeleton sea squirts and they look like they are right out of a horror movie! There isn't much info out there about these guys, but reading up about sea squirts I learned that they begin life resembling tadpoles with tails, eyes, mouths, hearts and backbones (meaning they are vertebrates) and as they mature they lose most of that... So maybe turn into inverts? What weird creatures... Anyway this seems to fit right into Halloween
