Why do Hydra end up with just a single head?
(PHYSorg.com Biology)

Often considered immortal, the freshwater Hydra can regenerate any part of its body, a trait discovered by the Geneva naturalist Abraham Trembley nearly 300 years ago. Any fragment of its body containing a few thousands cells can regenerate the entire animal The one-centimeter polyp has a developmental organizer center located at the head level, and another located in the foot. The head organizer performs two opposite activities: activating, which causes the head to differentiate, and inhibiting, which prevents the formation of supernumerary heads.