The bell of Acromitus flagellatus is hemispherical. The color of the bell is generally white, occasionally featuring irregular dark spots. The exumbrella is either smooth or finely granulated. The oral arms are slender, approximately as long as the bell’s diameter, and have long, thread-like endings that carry short filaments. The sub-genital papillae are shaped like hammers or hearts. There is a richly branched intracircular anastomosing network.