Any of various primitive eellike vertebrates of the class Agnatha, such as a lamprey, lacking jaws and true teeth and having a circular, sucking mouth.

• 圆口纲脊椎动物一种类似鳗鱼的无颚纲原始脊椎动物，如盲鳗，没有下颚和真正的牙齿，而长着圆形吮吸的嘴