Agnatha

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Agnatha ("no jaws") is a paraphyletic superclass of jawless fish in the chordate, subphylum of the vertebrates.

[change] Characteristics

Agnatha have existed since the Cambrian, and continue to live in modern times. There are two extant group of jawless fish, the lampreys and the hagfish, with about 100 species between them. In addition to the absence of jaws, the Agnatha are characterised by absence of paired fins, the presence notochord both in larvae and adults; and seven or more paired gill pouches. The bronchial arches supporting the gill pouches lie close to body surface. There is a light sensitive pineal eye. There is no identificable stomach. The Agnatha are ectothermic, with a cartilaginous skeleton, and the heart contains 2 chambers.

[change] Fossil agnathans

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.

[change] Groups

Myxini (hagfish)
Hyperoartia

• Petromyzontidae (lampreys)

Pteraspidomorphi
Thelodonti
Anaspida
Cephalaspidomorphi

• Galeaspida
• Pituriaspida
• Osteostraci