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Opheliids are infaunal, deposit-feeding worms found in a wide range of habitats from sandy to muddy sediments and from intertidal regions down to deep sea. Some species are easily noticed when disturbed, exhibiting fast, energetic movements superficially similar to nematodes. Opheliids are typically elongated, slender bodies with pointed prostomial. All opheliids have a distinctive ventral groove along part or all of the body. Internally, some opheliids are unusual in lacking circular muscles (Blake, 2000; Rouse & Pleijel, 2001).
Opheliidae currently consists of two subfamilies—Ophelininae and Opheliinae; the third subfamily—Travisiinae—was recently moved from Opheliidae to Scalibregmatidae (Paul et al., 2010). Considerable morphological and behavioral differences are found between these two clades (Law et al., in review). Whereas members of Opheliinae (e.g. Thoracophelia mucronata) exhibit typical peristaltic behavior found in many muddy sediment burrowers, members of Ophelininae (e.g. Armandia brevis, Ophelina acuminata) lack both circular muscles and eversible anterior regions necessary for peristalsis and thus exhibit undulatory burrowing behavior.