= The simplest multicellular species
One of the simplest known seems to be: https://en.wikipedia.org/wiki/Trichoplax
https://www.u-tokyo.ac.jp/focus/en/articles/a_00220.html "The simplest multicellular organism unveiled" from 2013 mentions Tetrabaena socialis.
Then of course: <Caenorhabditis elegans> is a relatively simple and widely studied <model organism>.
\Video[https://www.youtube.com/watch?v=1v6cgSkiHik]
{title=Nicole King (UC Berkeley, HHMI) 1: The origin of animal multicellularity by iBiology (2015)}
{description=
* https://youtu.be/1v6cgSkiHik?t=513 <multicellularity is polyphyletic>, e.g. evolved separately on <plants>, <fungi> and <animals>.
* https://youtu.be/1v6cgSkiHik?t=668 describes how <unicellular organism> choanoflagellates form <colony (biology)>, and how animals are characterized by certain key types of cellular interaction: adhesion, communication, regulation (cell differentiation) and extra cellular matrix production
}