Evolution of translucent worms

source: https://en.wikipedia.org/wiki/Symsagittifera_roscoffensis#/media/

In northwestern France, green patches on the beach look like seaweed but are actually translucent worms. Green algae lives inside them, making them appear green. This worm serves as a greenhouse for the algae, and the algae provides food for the worm. It is hard to tell where one begins and the other ends. The evolution of this creature is an example of how symbiogenesis, or how species evolve in cooperation with one another. The scientific name of this worm is Convoluta roscoffensis, but the Englishman who first described them called them "plant-animals."