Primo, the cyclostomes, whose jaws are fused into a flexible ring and whose gill openings are simply a large number of holes, an order consisting of only one family. Example: the lamprey.

第一目，圆口鱼，鳃合成为一个转动的圈环，鱼鳃开合有许多小孔，这一目只有一科。类型：人目鳗。