Hydra are predators. Their tentacles are like jellyfishes; they have little stinging barbs (cells called Cnidocytes) that when touched, expel a poisonous thread towards the prey. The prey usually trigger many Cnidocytes at once, resulting in a fatal dose of neurotoxins.
This video shows a Hydra hunting and eating a Daphnia- ENJOY
Hydra with tentacles:
Close up of hydra tentacle (end of it)