Anguilliformes

Anguilliformes is an order of ray-finned fishes commonly called eels. Its members have elongated bodies and reduced or absent pelvic fins.

Actinopterygii: Actinopterygii is the largest class of fishes, distinguished by fins supported by bony rays. Anguilliformes belongs to this broad group of ray-finned fishes.
Anguilliform locomotion: Anguilliform locomotion is swimming by propagating waves of lateral body bending from head to tail. Eels use this whole-body wave to move through water and confined habitats.
European eel: The European eel is a catadromous eel native to European and North African waters. Its migration between continental waters and the Sargasso Sea exemplifies a complex eel life cycle.
Eel-shaped fish: Eel-shaped fish are fishes with long, slender bodies, including both true eels and unrelated groups. The body shape alone does not establish membership in Anguilliformes.
Elongated body: An elongated body is a body shape in which length greatly exceeds width or depth. This characteristic shape gives eels their familiar profile and aids movement through narrow spaces.
Eel migration: Eel migration is the movement of eels between habitats, including long-distance movements between freshwater and the sea. Migration connects feeding habitats with breeding grounds in several eel life cycles.
American eel: The American eel is a catadromous species found in freshwater and coastal habitats of eastern North America. It shares a Sargasso Sea spawning region with the European eel.
Lamprey: Lampreys are jawless vertebrates with round, sucker-like mouths and no paired fins. Their eel-like bodies can invite confusion, but lampreys are not ray-finned fishes.
Pelvic fin: A pelvic fin is a paired fin on the underside of a fish, usually behind the pectoral fins. Pelvic fins are reduced or absent in the order, a useful feature of its body plan.
Fish metamorphosis: Fish metamorphosis is a developmental transformation in which a larva changes markedly in form and ecology. Leptocephali transform into glass eels or other juvenile forms before adult life.

Linked from 6 pages