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By BS MediaTwitter Profile | Published: Wednesday, 28 June 2017
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Sea Lampreys
Sea Lampreys
The class Cyclostomata is divided into two orders:

1) Petromyzonia and 2) Myxinoidea
Order 1: Pefromyzonia
  1. Lampreys are included in this order.
  2. The buccal funnel is suctorial and shows horny teeth.
  3. The mouth is present in the buccal funnel.
  4. The nasal sac is dorsal. It has no connection with the pharynx.
  5. Eyes are functional.
  6. Seven pairs of gill slits are present.
  7. A well developed dorsal fin is present.
  8. Branchial basket is complete.
  9. Brain is well developed.
  10. Pineal eye is well developed.
  11. Ear has two semicircular canals.
Ex: 1. Petromyzon (Sea-lamprey) Lampetra fluviatilis.
Order 2: Myxinoidea
  1. It includes the hag-fishes or slime eels.

    hagfish Myxini 218

  2. Buccal funnel is absent.
  3. The nasal sac open into pharynx, through a canal.
  4. Eyes are vestigial organs.
  5. Dorsal fin is absent or very small.
  6. Branchial basket is poorly developed.
  7. Brain is primitive.
  8. Pineal eye is reduced.
  9. Ear has only one semicircular canal.
  10. The hag-fishes are all marine.
Ex: 1. Myxine glutinosa (Hag-fish or slime eel). 2. Eptatretus (Bdellostoma)
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