Agnatha (means jawless): The Class Agnatha represents the most primitive 'jawless' vertebrates. MHC genes have been cloned from all vertebrate classes except Agnatha.
Agnatha
Lampreys and hagfishes are the only jawless vertebrates to survive today. They both have a round mouth and for this reason are often referred to as cyclostomes. They are the most primitive of the vertebrates.
agnathan
(ag-naa-thun)
A member of a jawless class of vertebrates represented today by the lampreys and hagfishes.
agonistic behavior ...
Agnatha, jawless fish. This group contains two significant animal groups, hagfish and lampreys. yet their nearest common ancestor is the ancestor of all vertebrates, so Agnatha is paraphyletic.
Agnatha A superclass of vertebrates whose members lack jaws and paired appendages and possess a cartilaginous skeleton and a persistent notochord. Lampreys and hagfishes.
Agonist The "prime mover" or muscle of reference.
This includes the bony fish (Osteichthyes), the cartilaginous fish (Chondrichthyes) such as sharks and rays, and the jawless fish (Agnatha).
Also included are the surviving fishes: jawless (Agnatha), cartilaginous (Chondrichthyes), and bony (Osteichthyes). The other classes up the evolutionary ladder are the amphibians, reptiles, birds and mammals.
Class Amphibia ...
Their skeleton is made of of cartilage, as is that of sharks. Hagfish lack jaws, and for this reason used to be classified with the lampreys in a group called the Agnatha ("no jaws") or the Cyclostomata ("round mouth").
 See also: Class, Organ, Species, Animal, Human
  
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