Agnatha
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| Agnatha | ||||||||
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Myxinoidea (hagfish) |
Agnatha (Greek, "no jaws") is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. There are about 60 modern species divided into Lampreys and Hagfish. 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 branchial 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, and the heart contains 2 chambers. Individual hagfish are hermaphroditic, with both ovaries and testes, but the female gonads remain non-functional until the individual has reached a particular stage in the hagfish lifecycle. Hagfish do not have a larval stage, in contrast to lampreys, which have a long larval phase.
Lampreys are reasonably successful parasitic predators. They attach themselves to other marine animals and abrade a hole through the skin with their rasp-like tongue in order to attack the underlying tissues. Modern lampreys always spawn in fresh water although many spend at least part of their life in the sea. Hagfish are marine and alternate between functioning as scavangers, parasites, and as active predators consuming marine worms.
Although a minor element of modern marine fauna, Agnatha were prominent among the early fish in the early Paleozoic. Two types of an Early Cambrian animal with apparent fins, vertebrate musculature, and with 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.
The first armored agnaths—the Ostracoderms, precursors to the bony fish and hence all tetrapods, including human beings—are known from the middle Ordovician, and by the Late Silurian the agnaths had reached the high point of their evolution. Agnaths declined in the Devonian and never recovered.
Modern agnathids generally have cartilaginous skeletons. Many Ordovician, Silurian, and Devonian agnathids were armored with heavy bony plates. Neither modern nor suspected Cambrian agnathids were/are armored.
In the current cladistic system, the class or superclass Agnatha is not recognised, as it a paraphyletic taxon.
External Links:
- http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/units/fish2.htm
- http://www.museum.nrc.gamagori.aichi.jp/interfa/ATLASE/AGNATH.HTM
- http://www.cyhaus.com/marine/agnatha.htm
- http://oceanlink.island.net/oinfo/hagfish/hagfish.html
- http://tolweb.org/tree?group=Hyperoartia&contgroup=Vertebratade:Kieferlose