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Swedmark, 1952

Description:
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.

Size:
Length about 6 mm for 37 segments.

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

Habitat:
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.

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

Psammodrilus balanoglossoides