A graceful sea anemone with long tentacles arranged in multiples of six. It grows up to 12 cm tall and 6 cm across the base which is strongly adherent. The disc and tentacles are translucent pale greyish in colour, while the disc is variegated with cream, and with a regular but not striking pattern. The tentacles have lateral dark lines down their length. The column is tall in extension with no suckers and pale yellowish buff in colour, with regular vertical stripes of brown flecks of variable intensity.
•Column at full extension up to 12 cm high and 2 cm diameter.
•Oral disc wider than column, with an unusually wide mouth.
•Tentacles up to 200, long, slender, arranged hexamerously.
•Striped pattern on column is consistent and distinctive.
Sagartiogeton undatus is often found in the company of Sagartia troglodytes or Cereus pedunculatus, where these occur buried, and in the past has been confused with both these species although easily distinguished from them by its lack of suckers. Philip Henry Gosse christened this species the Snakelocks, but this name proved popular with the more common species Anemonia viridis.