I think you misread wikipedia when it talks about its endosymbioses. Whole bacteria are found within an organlle (the endoplasmic reticulum) of Trichoplaxs.

That being said what you described does happen in a number of organisms (including 'complex' ones like nudibranchs): they steal the chloroplasts from the algae they eat in a process called kleptoplasty. Seeing as mitochondria and chloroplasts originated as bacterial endosymbionts that were then heavily integrated into their hosts, calling kleptoplasty a form of symbiosis isn't that unusual.

Fun fact: Animal embryos can be disassociated by depriving them of calcium (E-cadherin, the molecule that holds the cells together, needs to calcium to work) and then can be allowed to reassociate by adding back calcium. If you do this in early enough stages then the embryo will function and develop normally once reaggregated, despite all the cells being jumbled up

"peptide-based protocol" is a pretty good band name

Thank you for the summary. I don't have time to go down a rabbit hole at the moment, so this was just enough to sate my curiosity until I do have time.

ISTR you can do the sieve thing with true living sponges, too. Life on earth is wild. I wonder if it will be considered mild once we find some interesting life off-planet.

Fucking interesting!