Petromyzontiformes

Article By:

Boschung, Herbert T. Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama.

Last reviewed:January 2021

DOI:https://doi.org/10.1036/1097-8542.503800

An order of jawless fishes commonly known as the lampreys. Members of the order Petromyzontiformes (class Petromyzontida or Hyperoartia) are known as lampreys. Characteristics of these jawless fishes include the following combination of features that describe the order: eel-like body; no bones; no paired fins; one or two dorsal fins; diphycercal (isocercal) tail (that is, with symmetrical upper and lower lobes) in adults, hypocercal (that is, with a larger lower lobe) in ammocoetes (larval lampreys); no barbels; teeth on oral disc and tongue (Fig. 1); two semicircular canals; seven pairs of external lateral gill openings; well-developed eyes in adults, and rudimentary eyes in ammocoetes, usually laterally placed; single median nostril (a nasohypophyseal sac with an external opening only); spiral valve and cilia in intestinal tract; small cerebellum; separate sexes; and oviparous, with small and numerous eggs. The ammocoetes undergo radical metamorphosis, and all species spawn in freshwater. See also: Jawless vertebrate

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