Lampreys are fish with primitive features. Together with hagfish, they are the last surviving jawless vertebrate. These creatures don't exist in my home country, so I've only known their existence from encyclopedia back in elementary school. Their somewhat scary circular mouth is full of teeth and will surely etch into memory.
Some time ago, Bodo posted a transition unit challenge in Origami Design School Discord server. I solved them, and thanks to that, my understanding of the construction improved. I'll share the screenshot of the challenge so you can try solving it.