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。

Triops is a genus of small crustaceans in the order Notostraca (tadpole shrimp). Some species are considered living fossils, with a fossil record that reaches back to the late Carboniferous, 300 million years ago. One species, Triops longicaudatus, is commonly sold in kits as a pet. Most Triops have a life expectancy of up to 90 days, and can tolerate a pH range of 6–10.