Aspidosiphon Diesing

Diagnosis: Introvert usually longer than trunk. Recurved hooks in numerous rings (absent in three species, scattered in two). Trunk with anal shield composed of hardened units that may be inconspicuous. Introvert protrudes from ventral margin of shield. Body wall either with continuous longitudinal muscle layer or with longitudinal muscle layer separated into anastomosing, sometimes ill­defined bundles. Small tentacles enclose dorsal nuchal organ but not mouth. Contractile vessel without villi. Two introvert retractor muscles may be almost completely fused. Spindle muscle attaches posteriorly. Two nephridia. One species may exceed 100 mm, but most are less than 40 mm long. Commonly inhabiting some sort of shelter, e.g. holes in coral or soft rock, mollusc shells, or arenaceous foraminiferan tests.