Agnatha

Agnatha is a traditional grouping of jawless vertebrates, including living lampreys and hagfishes and extinct armored forms. Its membership and rank vary among classification schemes.

Vertebrate: An animal with a backbone or an embryonic structure that develops into one. Agnathans are vertebrates, despite lacking the jaws characteristic of most living vertebrate lineages.
Lamprey: An eel-shaped jawless vertebrate with a circular mouth, found in freshwater and marine environments. Lampreys are living agnathans with a distinctive toothed oral disc.
Suction feeding: A feeding method that draws prey or food into the mouth by generating negative pressure. Lampreys use suction to attach to hosts, while other jawless forms feed differently.
Gnathostome: A vertebrate belonging to the jawed-vertebrate lineage, including most living fishes and all tetrapods. Gnathostomes possess jaws, the major anatomical distinction used to contrast them with Agnatha.
Jaw: A hinged skeletal structure that supports biting or grasping in many vertebrates. The absence of jaws is the defining trait behind the name Agnatha.
Hagfish: A marine jawless craniate that produces copious defensive slime and feeds on dead or dying animals. Hagfishes are living jawless vertebrates whose placement has shaped debates about Agnatha.
Parasitism: An interaction in which one organism benefits while living on or in a host that is harmed. Some lampreys attach to fish and feed on their body fluids and tissues.
Cyclostomata: The living jawless vertebrate group comprising lampreys and hagfishes. Cyclostomata is a narrower modern grouping for the two living lineages often included in Agnatha.
Craniate: A chordate with a skull or comparable cranial structure. Hagfishes and lampreys possess skulls, placing them among craniates even without jaws.
Ostracoderm: An informal name for several groups of extinct, often armored jawless fishes. Ostracoderms are prominent fossil representatives of traditional Agnatha.

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