Starfish are more interesting than they seem at first glance. Most know of their LEGENDARY ability to regenerate - not only to regrow new arms, but severed arms can regrow a whole new starfish! - but that's not the only trick they hold. https://twitter.com/RebeccaRHelm/status/1356610600743354371

Echinoderms - a phylum of animals containing starfish, sea urchins, sea cucumbers, and sea lilies - are unique among animals for their RADIAL (usually 5fold) symmetry. But their larvae start their lives with a quite ordinary bilateral body plan. How does the symmetry break?

After passing through several larval stages, the adult radially symmetric body starts forming completely anew, on the side of the main bodily cavity (as seen in the video quoted in OP)! This new body eventually overgrows and consumes the larva.

This adult body has an anatomy like no other. Most strikingly, echinoderms lack a central nervous system - it instead functions a fully distributed network. And yet, despite bizarreness, molecular studies place echinoderms as a cousin clade to vertebrates...

So, how does the starfish regenerate so spectacularly? Having no central nervous system helps - but more helpful is that all its cells are equipped with ability to divide indefinitely, without need for stem cells.

Usually, such cells are found in cancers...

But the starfish can regulate this kind of growth. The similarity has led some biologists to try using the enzymes involved in regulation in blocking cancer cell growth - and some experiments turned out successful.

So, what are the starfish? Can they be understood as a cancerous growth - but one that has successfully achieved self-sustenance, self-regulation, and even sexual reproduction? Is the adult starfish body a tumour that outgrows its former larval host, taking on a life of its own?