They are used to feel around and to capture prey. They extent their tentacles to capture prey, and use their nematocysts (nettle-like cells on their tentacles) to pierce and poison their prey. Hope that helps!
The polyp uses its tentacles to capture and ingest the phytoplankton it eats. The tentacles are also the primary means of defense for the polyp. They conceal the stinging nematocyst cells. The tentacles are also used to clear away debris from the mouth of the polyp.
