Lophogastrids appear to be most closely allied to the strictly fossil pygocephalomorphs; indeed, species recognizable as lophogastrid relatives are known from the Pennsyl-vanian Period, about 325-286 million years ago. In these groups the sternites (ventral shields) are wide, with the thoracic legs being located on the outer edges of the somites.
The order Lophogastrida contains two extant families, the Lophogastridae with six genera, and the Eucopiidae with the single genus Eucopia. Many scientists now regard lophogastrids as a suborder of the order Mysidacea. They differ from the other mysid suborder in having gills on most thoracic appendages; no statocyst (organ governing the sense of balance) in the uropod; and no modifications of the anterior pleopods in the males. Some authors have suggested that any resemblance to other mysids is purely superficial and that the lophogastrids should have their own order. The latter classification is followed here.
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