There are only two species — Seison annulatus and S. nebaliae — that live as epibionts on crustaceans of the taxon Nebalia (Leptostraca, Malacostraca). As microscopic organisms of 1–2 mm in length their appearance is unmistakable, with a weakly swollen head, extremely long neck with sections that retract like a telescope, a thickened trunk, and the foot with a terminal adhesive disk (Fig. 17A). Equally unique are the slow waving movements over the host while being anchored by the foot — and occasional change of location in the manner of a spanworm.
KeywordsVentral Valve Ciliary Band Ground Pattern Cement Gland Planktotrophic Larva
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