Lichen is not a plant nor an animal - primitive or developed.

Lichen is the result of a relationship - a synergistic relationship between algae and fungi. Without the relationship lichen would not exist. Now is that complicated or what?

Synergism is a cooperative action that results in something greater, albeit different, than each of the cooperating agents. To get a better description or understanding than that, take a biology class. The local Community College will be happy to oblige.