They're TENTACLES, not ARMS

Hydra are freshwater micro-invertebrates that belong to the phylum Cnidaria, which include jelly fish and sea anemones. Like all Cnidarians, Hydra too have nematocysts or stinging cells at the tips of their tentacles. The tentacles and nematocysts are used to capture prey. Hydra are carnivorous. They feed on other microinvertebrates such as, daphnia and rotifers. When conditions are favorable, Hydras undergo asexual reproduction through a process called "budding". However, these organisms can also be hermaphroditic. The egg is typically found towards the base of the animal. Testes are typically found superior to the ovary. You can see the reproductive structures in the image here. On its right side, you can see two growths forming. When the growths mature, the testes wall will burst, releasing free swimming sperm cells, that will make their way to the ovary and fuse with with egg cell.