Distribution: Circumtropical; intertidal and shallow subtidal; in crevices of coral rubble or soft rock; very common. Characteristics: Trunk covered with dark, dome-shaped papillae. Trunk length usually between 20 and 40 mm. Introvert longer than trunk; with dark pigment bands and over 100 rows of hooks on introvert. About 20 short tentacles. Although the two Phascolosoma species in Bocas are easily distinguished by gross morphology, it is usually necessary to study the morphology of the introvert hooks with light microscopy in order to identify Phascolosoma species.