Agnatha (Greek, "no jaws") is a paraphyleticsuperclass of jawless fish in the phylumChordata, subphylum Vertebrata. Agnatha have existed since the Cambrian, and continue to live in modern times. There are two extant groups of jawless fish (sometimes called cyclostomes), the lampreys and the hagfish, with about 100 species between them. In addition to the absence of jaws, Agnatha are characterised by absence of paired fins; the presence of a notochord both in larvae and adults; and seven or more paired gill pouches. The bronchial arches supporting the gill pouches lie close to the body surface. There is a light sensitive pineal eye (homologous to the pineal gland in mammals). There is no identifiable stomach. Fertilization is external. The Agnatha are ectothermic, with a cartilaginousskeleton, and the heart contains 2 chambers.
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(Class Agnatha): The most primitive Vertebrates evolved about 500 Mya. The extant species hagfish and lampreys in the order Cyclostomata belong to this group.
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