Trichoplax

From the "favorite organisms" file, Pharyngula has an article on Trichoplax and what it might tell us about the evolution of multicellular creatures (particularly metazoans, aka animals). Trichoplax is the simplest organism that we can call an animal -- it doesn't have tissues or organs, but it has differentiated cells and versions of the genes that determine body layouts in animals. They're sort of halfway between amoebas and jellyfish. (There's an earlier article about Trichoplax on Pharyngula as well.)