Tunicates evolved long before animals that you would recognize as typical vertebrates. In their free-swimming larval state, tunicates do have a "notochord", or nerve chord, running along their head-tail axis. This structure is a kind of proto-spine. The evolution of true vertebrates proceeded from this condition by neoteny, the retention of larval traits in the adult body plan. Pyrosomes look so bizarre compared to a layman's notion of what a chordate should look like because they are colonial aggregations of lots of individual adult tunicates that have resorbed their notochords (I'm pretty sure that's the case, despite what the photo description says).
throwhauser|4 years ago
Thanks for the explanation! Slightly less freaked out now.