Agnatha

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Agnatha
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Vertebrata Class: Agnatha
Orders

Agnatha is a class of jawless fish in the phylum Chordata. They are in the Sub-Phylum Vertebrata. There are about 60 modern species. The name is from the Greek and means "no jaws". They are divided into Lampreys and Hagfish. In addition to the absence of jaws, Agnatha are characterised by absence of paired fins; presence of a notochord both in larvae and adults; 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. There is no identifiable stomach. Fertilization is external. Individual hagfish are hemaphrodorphic with both ovaries and testes, but the female gonads are not functional. Lampreys are bisexual. Hagfish do not have a larval stage whereas Lampreys 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. Agnathids were well established by the late Ordovician and are found in the Silurian as well. Agnathids declined in the Devonian and never recovered.

Modern agnathids generally have cartilaginous skeletons. Ordovician and Silurian agnathids were armored with heavy bony plates. Neither modern nor suspected Cambrian agnathids were/are armored.

External links:

• http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/units/fish2.htm
• http://www.kheper.auz.com/gaia/biosphere/vertebrates/agnatha/
• 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=Vertebrata