背甲目（学名：Notostraca）及其单一科鲎虫科（学名：Triopsidae），俗称“恐龙虾”、“蝌蚪虾” 或“盾虾”。 其下鲎虫属（Triops）和冷鲎虫属（Lepidurus）两属，自三叠纪以来在外形上都没有发生过显着的变化，均被视为活化石。鲎虫有着宽阔、平坦的背甲，甲下埋藏着鲎虫的头部，头上有一对复眼。鲎虫的腹部很长，呈节状，生有多对扁平足。尾节的两侧是一对细长的尾叉。群间的表型可塑性为种间的辨别带来了困难，而繁殖模式上的变异进一步加深了这种困难。鲎虫性杂食，生活在暂时性水池和浅塘的底部。

The order Notostraca comprises the single family Triopsidae, containing the tadpole shrimp or shield shrimp. The two genera, Triops and Lepidurus, are considered living fossils, having not changed significantly in outward form since the Triassic. They have a broad, flat carapace, which conceals the head and bears a single pair of compound eyes. The abdomen is long, appears to be segmented and bears numerous pairs of flattened legs. The telson is flanked by a pair of long, thin caudal rami. Phenotypic plasticity within taxa makes species-level identification difficult, and is further compounded by variation in the mode of reproduction. Notostracans are omnivores living on the bottom of temporary pools and shallow lakes.