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(Muller, 1788)

Description
Diameter of base up to 60 mm, height in full extension to 120 mm or more.
Base wide, usually wider than the disc, and firmly adherent, its outline relatively smooth as basal laceration does not occur (cf. Sagartiogeton laceratus). Column very tall in full extension, cylindrical, flaring out abruptly just below the disc; when tightly contracted it may become remarkably flat, only a few mm thick. Maximum extension of the column normally occurs only in darkness or when the anemone is buried. A loose investment of mucus and detritus occasionally occurs on the lower part of the column; small solid papillae may also be present but these are rare in British specimens. Cinclides present in two regions: just above the limbus and on the distal part of the column, usually discernible as conspicuous dark dots on the pale endocoelic stripes. Acontia emitted only with great reluctance. Disc moderately wide and flattish; as in S. laceratus the mouth-slit is usually long, with a characteristic slight gape.
Tentacles: Long or very long and gracefully displayed, hexamerously arranged with those of cycles I and 2 almost equal, up to 192.
Colouration: Column pale yellowish buff, finely freckled with brown. A series of pale cream endocoelic stripes runs upward from the limbus, those on the primary and secondary endocoels reaching almost to the top of the column, the rest fading away a short distance above the limbus. Disc translucent, pale grey or brown, usually with narrow opaque cream lines on the mesenteric insertions. A pattern of greyish or brownish markings is invariably present often ill-defined and rarely as sharply marked as in S. laceratus; it often incorporates a ring of elongated cream spots running mid-way between the mouth and the tentacle bases. Tentacles very translucent, pale grey, with a pair of dusky longitudinal lines on the oral face and usually with one or two diffuse cream bars near the base. In full expansion the tentacles form a delicate, diaphanous and very graceful spray.
The colouration of the disc is often broken up by wedges of opaque black or brown, usually irregular in arrangement and occupying several adjacent radii, the colour often spreading on to the related tentacles. Otherwise little variation occurs, apart from slight differences in tone and intensity.
Reproduction: This species does not appear to reproduce asexually, hence all specimens are regularly hexamerous, nor has viviparity been reported.
Nematocysts of acontia: p-mastigophores 34-54 x 3.9-7.5 öm, b-mastigophores 19-34 x 2.0-4.0 öm.

Habitat
Typically found buried in mud, sand, or gravel, attached to a stone or shell about 10-15 cm below the surface. Also amongst rocks, under stones, on worm tubes, etc., usually in shaded places. Occurs from the lower shore to depths of at least 100 m.

Distribution
Frequent to common on all British coasts although rarely present in abundance in any one locality. Widespread throughout western Europe from Scandinavia to the Mediterranean.

Remarks
Although a fairly common species - specimens can be found in most localities - this anemone is surprisingly poorly known. It is often found in the company of Sagartia troglodytes or Cereus, where these occur buried, and in the past has often been confused with both these species although easily distinguished from them by its lack of suckers.
The common name "Snakelocked anemone" was originally applied to this species, probably by Gosse, but has subsequently been usurped by Anemonia sulcata, for which species the name is more appropriate.

Sagartiogeton undatus