CYCLOSTOMATA
The cyclostomata (Gr.,cyklos,circular+stoma,mouth)are the living agnathans, they are primitive in many respects,but specialized in others.They are a modified degenerate offshoot of the primitive vertebrate stalk,they arose in the Devonian.They have round bodies with laterally compressed protocercal tail,they resemble eels superficially.The suctorial mouth is ventral and round (hence Cyclostomata).They are the only living vertebrates without jaws.Gill chambers are round pouches (hence Marsipobranchii).In the adult state they are parasitic or scavengers on fishes.Cyclostomata include lampreys and hag-fishes,its two orders are not closely related,because they have evolved from different orders of ostracoderms.
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