tunicates

Adult tunicates have a hollow cerebral ganglion, equivalent to a brain, and a hollow structure known as a neural gland.

What's fascinating here is that tunicates (like sea squirts) are thought to be the closest relatives to vertebrates, closer even than amphioxus.

I thought it was tunicates (sea squirts) because they are closely related to vertebrates and start off free swimming with a notochord.

Incidentally, of the marine invertebrates, the ones we are most closely related to are tunicates and salps, which are blobs that filter feed [2].

It freaks me out that tunicates are chordates.

Some tunicates actually concentrate vanadium for oxygen transport.

Therefore the evolution of most mobile animals from sponges is far more unlikely than the evolution of sponges from ctenophore-like ciliated animals, which is a kind of evolution that has been seen in a very large number of examples in other animals, for instance in tunicates.

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).

In fact, insects and crabs are as close as we are to Tunicates [0]: the same phylum, Chordata for us or Arthropoda for them, but different Subphylums (Vertebrata vs Tunicata for us, Insecta vs Crustacea for them).

They occupy less than 0.1% of the world's ocean area, about half the area of France, yet they provide a home for at least 25% of all marine species,[4][5][6][7] including fish, mollusks, worms, crustaceans, echinoderms, sponges, tunicates and other cnidarians.[8] Coral reefs flourish in ocean waters that provide few nutrients.

In their free-swimming larval state, tunicates do have a "notochord", or nerve chord, running along their head-tail axis.

Tunicates and Echinoderms seem to have evolved from bilateral ancestors like the ancestors of chordates.

Tunicates evolved long before animals that you would recognize as typical vertebrates.