The hydra is one of the few creatures on the planet that can literally pull itself together. Even if a hydra is blown to bits -- by an attacking predator, for example -- its remaining cells form new bodies, complete with heads, mouths and tentacles.
As Rebecca Helm writes at The Conversation:
Slowly, the disembodied pieces will begin crawling together, rising like tiny volcanoes from the sea of shredded remains. Forms will begin to take shape. Mouths and skinny tentacles stretching out into the water, and suddenly little bodies everywhere have regrown.