Article
Article
Hexanchiformes
Article By:
Boschung, Herbert T. Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama.
Last reviewed:January 2020
DOI:https://doi.org/10.1036/1097-8542.803330
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An order of squalomorph sharks commonly known as the six- or seven-gill sharks. The order Hexanchiformes comprises two extant families (Chlamydoselachidae, the frill sharks; and Hexanchidae, the cow sharks), four genera, and six species. These sharks are distinguished by a combination of the following characters: six or seven gill slits, all anterior to the pectoral fins; a single dorsal fin without a spine; anal fin present (the only order of squalomorphs with an anal fin); the dorsal, anal, and pelvic fins placed far posteriorly; eyes lacking a nictitating fold or membrane (at the inner angle of the eye or below the eyelid and capable of extending over the eyeball); spiracles present, but small and well behind the eyes; and ovoviviparous development. See also: Elasmobranchii; Selachii
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