The phylum of Cnidaria is one of the first branches in the tree of animal life to provide crucial insights on the evolution of immunity. Cnidarians are diblastic aquatic animals with radial symmetry and they are the simplest multicellular organisms that have reached the level of tissue organization. The renewed interest in the study of immunity in Cnidaria has led to additional information to the scenario of the first stages of immunity evolution revealing the cellular processes involved in symbiosis, in the regulation of homeostasis and in the fight against infections. We investigated the inflammatory response in Cnidarian following injection of various substances different in type and dimension, and observed clear, strong and specific re...