At some point in our lives, we have all been through that (=embryonic cleavage).
Our paper on the unique development of a ribbon worm and the evolution of larval forms: https://evodevojournal.biomedcentral.com/articles/10.1186/s13227-015-0023-5
See more about Lineus ruber.
I wrote a text about the development and evolution of the turtle shell on The Node: Turtles in a nutshell.
It shows the beginning of shell formation in embryos and how this can help us understand the evolution of such unique body pattern. 3D animations and fossils are included!
Never thought I would post a vertebrate on this website… but having shoulders inside the rib cage made me make an exception, poor things :p