Hydra is a genus of freshwater cnidarians that includes more than 30 species that maintain the polyp structure throughout their lives and can measure between 2 and 20 millimeters in length (this last measurement corresponds to a fully extended polyp). The number of tentacles is highly variable (between 1 and 12) and all of them are covered in stinging cells called cnidocytes, capables of paralyze prey when they come into contact with them. These prey are usually small organisms (insect larvae, p