Agnatha

Agnatha is a paraphyletic infraphylum of jawless vertebrates within the subphylum Vertebrata, encompassing organisms known from as far back as the Cambrian, roughly 518 million years ago. Living representatives are the cyclostomes, hagfishes and lampreys, together numbering about 120 species, while extinct members include conodonts, cephalaspidomorphs, and various armored ostracoderms. As their name indicates, agnathans lack jaws and paired fins; they retain a notochord in both larval and adult stages and have seven or more paired gill pouches, cartilaginous skeletons, and two-chambered hearts. The group is explicitly recognized as paraphyletic: it cannot be assembled into a natural, complete grouping without either excluding some living cyclostomes or including all vertebrates, a tension that continues to drive debate over jawless-vertebrate relationships.