G. O. Sars, 1865
Diagnosis (summarized from Fryer, 1987b)
Body short, with few segments, tagmosis mostly obscure, terminating in postabdomen (telson) armed with a pair of terminal claws. Postabdomen sometimes tilted ventrally but never reflexed and never articulated to trunk. Trunk and its appendages, but not head, enclosed by functionally bivalved, but hingeless and uncalcified, carapace.
Head short. No headshield. Labrum fleshy. Compound eyes fused to give single, median sessile eye with many lenses. Ocellus present.
Antennules tubular in female. Antennae biramous, natatory (secondarily uniramous in female of Holopedium).
Exopod and endopod each with 2 or 3 segments, between them armed with 13 (Penilia) to 27 natatory setae. Six pairs of trunk limbs (or thoracic limbs), all pre-genital and showing much serial similarity, but 6th pair reduced.
Formerly superfamily Sidoidea.