45 David J. Carroll and Stephen A. Stricker (eds.), Developmental Biology of the Sea Urchin and Other Marine Invertebrates, Methods in Molecular Biology, vol. 1128, DOI 10.1007/978-1-62703-974-1_4, © Springer Science+Business Media New York 2014 Chapter 4 Trichoplax adhaerens, an Enigmatic Basal Metazoan with Potential Andreas Heyland, Roger Croll, Sophie Goodall, Jeff Kranyak, and Russell Wyeth Abstract Trichoplax adhaerens is an enigmatic basal animal with an extraordinarily simple morphological organization and surprisingly complex behaviors. Basic morphological, molecular and behavioral work is essential to better understand the unique and curious life style of these organisms. We provide basic instructions on how Trichoplax can be cultured and studied in the laboratory emphasizing behavioral and cellular aspects. Key words Placozoa, Behavior, Immunohistochemistry , In situ hybridizations, Cloning, Chemotaxis 1 Introduction Trichoplax adhaerens is a species from the phylum Placozoa that has been the subject of considerable investigation in recent years. The newly emerged interest in this enigmatic creature arose from its basal position within the animal kingdom as well as its extraor- dinarily simple morphological organization ( for recent reviews see refs. [1, 2]). Superficially one might believe that Trichoplax is an amoeba ( for general overview of Trichoplax morphology see Fig. 1 and ref. [3]). Instead Trichoplax is reported to have 4–5 cell types and several thousand cells. More importantly, it shows a remark- able array of behavioral responses to its environment, reproduces both sexually [4, 5] and asexually [4] and defends itself chemically against predators [6]. It also relies on a broad array of developmen- tal and physiological signaling pathways for essential cellular and molecular functions [7]. “It is a riddle, wrapped in a mystery, inside an enigma” Winston Churchill, cited in Miller and Ball 2008