"Have you heard of a pyrosome?" asks Carin Bondar.
My immediate answer was, no; no, I haven't - but you know me, I'm always curious :) Turns out that pyrosomes - which look almost other-worldly in the video below - are colonial tunicates: the same taxonomic group as the perhaps-more-familiar sea squirts. And that means that they're in the same phylum as us, for both tunicates and mammals are chordates.
These are adult seasquirts (image from reefbuilders.com).