Uncovering the evolutionary history of innate immunity: the simple metazoan Hydra uses epithelial cells for host defence.

Bosch T C, Augustin R, Anton-Erxleben F, Fraune S, Hemmrich G, Zill H, Rosenstiel P, Jacobs G, Schreiber S, Leippe M, Stanisak M, Grötzinger J, Jung S, Podschun R, Bartels J, Harder J, Schröder J M (2009); Dev Comp Immunol., 33(4):559-69. doi: 10.1016/j.dci.2008.10.004