↵​ The most primitive vertebrates are (a) ostracoderms (b) cephalochordates (c) placoderms (d) cyclostomes        

​Cylostomata is the group of primitive vertebrates

Cyclostomata is a group of agnathans that comprises the living jawless fishes: the lampreys and hagfishes. Both groups have round mouths that lack jaws but have retractable horny teeth. The name Cyclostomata means round mouths. Their mouths cannot close due to the lack of a jaw, so they have to constantly cycle water through the mouth.

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