鳗类，鳗：无颚的各种原始水生圆口纲脊椎动物中的一种，包括七鳃鳗和盲鳗。
Any of various primitive aquatic vertebrates of the class Cyclostomata, lacking jaws and including the lampreys and hagfishes.
脊椎动物的起源是进化生物学研究的核心问题之一，目前人们普遍认为脊椎动物与无脊椎动物中的脊索动物起源于同一个原始祖先。
The origin of vertebrates has always been a core question in evolutionary biology. It is well accepted that vertebrates and invertebrate chordate derived from a common ancestor.
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