甲胄鱼 jiǎ zhòu yú ostracoderm

甲胄鱼【 jiǎ zhòu yú  ostracoderm】生活在距今4亿多年到5亿年间的古生代时期。它们中的大多数身体的前端都包着坚硬的骨质甲胄，形似鱼类，但没有成对的鳍，活动能力很差。同时也没有上下颌，限制了它的主动捕食能力，食物范围很窄，因而没有发展前途。甲胄鱼是现存圆口纲动物，如七鳃鳗的祖先，它们彼此有一定的亲缘关系。又有人说它没有后裔，因为甲胄鱼适于水底生活，而圆口纲则运动灵便，适于半寄生或寄生生活，它们之间不一定有直接的关系，而是来自共同的无颌类祖先。

Armored fish [Ji] ǎ Zh ò u y ú ostracoderm] lived in the Paleozoic era, more than 400million to 500million years ago. Most of them are wrapped in hard bone armor at the front of their bodies, which looks like fish, but they have no paired fins and have poor mobility. At the same time, there is no upper and lower jaw, which limits its active predatory ability. The food range is very narrow, so it has no future. Armored fish are the ancestors of existing round mouth animals, such as lampreys. They have a certain genetic relationship with each other. It is also said that it has no descendants, because the armored fish is suitable for underwater life, while the round mouth class is flexible and suitable for semi parasitic or parasitic life. They do not necessarily have a direct relationship, but come from the common jawless ancestor.