Oxytocin/vasopressin-like Immunoreactivity ... in Hydra (PubMed)
Abstract only. "Nerve cells have been found in hydra, which react with antisera to oxytocin, vasopressin and mesotocin. These nerve cells have a high density in the ectoderm of basal disk and tentacles," However, "...the antigenic determinant of hydra is not oxytocin or vasopressin. The presence of oxytocin/vasopressin-like material in coelenterates, shows that this family of peptides is of great antiquity."