The class Cyclostomata is divided into two orders:
1) Petromyzonia and 2) Myxinoidea
1) Petromyzonia and 2) Myxinoidea
Order 1: Pefromyzonia
- Lampreys are included in this order.
- The buccal funnel is suctorial and shows horny teeth.
- The mouth is present in the buccal funnel.
- The nasal sac is dorsal. It has no connection with the pharynx.
- Eyes are functional.
- Seven pairs of gill slits are present.
- A well developed dorsal fin is present.
- Branchial basket is complete.
- Brain is well developed.
- Pineal eye is well developed.
- Ear has two semicircular canals.
Ex: 1. Petromyzon (Sea-lamprey) Lampetra fluviatilis.
Order 2: Myxinoidea
- It includes the hag-fishes or slime eels.
- Buccal funnel is absent.
- The nasal sac open into pharynx, through a canal.
- Eyes are vestigial organs.
- Dorsal fin is absent or very small.
- Branchial basket is poorly developed.
- Brain is primitive.
- Pineal eye is reduced.
- Ear has only one semicircular canal.
- The hag-fishes are all marine.
Ex: 1. Myxine glutinosa (Hag-fish or slime eel). 2. Eptatretus (Bdellostoma)
