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Agnatha
From Wikipedia, the free encyclopedia
Agnatha (Greek, "no jaws") is a paraphyletic[citation needed] superclass of jawless fish in the phylum Chordata, 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 cartilaginous skeleton, and the heart contains 2 chambers.
Although they are superficially similar, many of these similarities are probably shared primitive characteristics of ancient vertebrates, and modern classifications tend to move the hagfish into a separate group (the Myxini or Hyperotreti), with the lampreys (Hyperoartii) being more closely related to the jawed fishes.
Although a minor element of modern marine fauna, Agnatha were prominent among the early fish in the early Paleozoic. Two types of Early Cambrian animal apparently having fins, vertebrate musculature, and gills are known from the early Cambrian Maotianshan shales of China: Haikouichthys and Myllokunmingia. They have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is Haikouella. A possible agnathid that has not been formally described was reported by Simonetti from the Middle Cambrian Burgess Shale of British Columbia.
Many Ordovician, Silurian, and Devonian agnathans were armored with heavy bony-spiky plates. The first armored agnathans—the Ostracoderms, precursors to the bony fish and hence to the tetrapods (including human beings)—are known from the middle Ordovician, and by the Late Silurian the agnathans had reached the high point of their evolution. Agnathans declined in the Devonian and never recovered.