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Swedmark, 1952

Body vermiform and cylindrical.
Head with well-developed with an acorn-shaped prostomium, distally tapering to a slightly rounded tip. Buccal region of nearly same length (0.4 µm); with an Y-shaped mouth in the anterior part of ventral side. Two sensory organs in posterior part on dorsal side. Pharyngeal collar with a conspicuous surface pattern of about 25 rows of polygonal or hexagonal cells.
Six thoracic segments, each provided with a pair of thread-shaped dorsal cirri, gradually decreasing in length posteriorly, with the three posterior pairs abruptly shorter and more closely situated to the mid-dorsum. Cirri each supported by a whip-like internal acicula.
Boundary between thorax and abdomen externally poorly delimited. About 30 abdominal segments, each with a pair of ventral transverse ridges containing a row of hook-like chaetae. Number of hooks per ridge varying from 16 in the most anterior segments to two or three in the most posterior ones. Hooks with long shaft and a large main tooth surrounded by smaller teeth.
Pygidium rounded, with sensory cilia and subterminal anus. Body completely ciliated except for pharyngeal region and a mid-dorsal furrow on the abdomen. Cilia on prostomium and cirri particularly active. Seven, segmental, transverse ciliary rings, of which most anterior one marks border between prostomium and peristomium; other six directly in front of the cirri, with especially long cilia. Prostomium with small apical ciliary tuft.

Length about 6 mm for 37 segments.

Transparent. Middle part of intestine green to dark-brown.

Intertidal sand flats, and shallow subtidal sandy sediments. Semi-sessile animals living inside a thin mucous tube that is covered with detritus and sediment particles.

Atlantic, North Sea (Sylt, Amrum, Husum area, Belgian coast), English Channel, Irish Sea, Baltic (Hohwachter Bucht), East coast of North America (Massachusetts).

Psammodrilus balanoglossoides