All I have to say is: glad I’m not a hagfish. : )

Tunicates have a free-swimming, bilaterally-symmetric larval stage, right? Something like a planktonic tadpole? I have to wonder if the first vertebrates were evolved from the juvenile forms of sessile species, retaining neotenic features and developing a spinal cord while they were at it.

And how do hagfish lay eggs? The same way I presumed babies were born when I was a kid: through their butts.

Maybe whatever intermediate forms existed evolved very quickly from invertebrates to vertebrates, leaving little time to speciate. Your last two hypotheses seem sound to me, too.

One other final delightful hagfish detail I forgot to mention in the main post: the sperm and eggs of hagfish have no special duct to help them on their merry way. Instead, they are released directly into the body cavity of the hagfish, from which they must find their own way out, generally through an opening in the digestive tract, if wikipedia is to be believed. At least sperm can swim, but how on earth do the eggs get out, except maybe by overwhelming force of numbers? Well, on the other hand, perhaps unlike sperm, who are sure they know where they are going, the eggs are willing to stop and ask for directions.

So, the point of the recent study is that hagfish are not the ancestors of today’s cartilaginous and bony fish? Too bad, I suppose, but at some point, “missing links” have got to be inferred, rather than found. (Especially when those things don’t have bones.) Can’t collect ’em all. If the hagfish isn’t the missing link, I wonder, instead, why the missing link went extinct. Perhaps there was no happy medium between the vertebrate and invertebrate that our jawless fishes couldn’t better exploit? Maybe it pays to choose to have a backbone, or not?