Myxini
The Greek word myxa means slime. They are known as hagfish. The ugly fish is called slime eel. Their number of species is 70.
1. Their bodies are slender, elongated, with slippery glands and without paired fins.
2. They look like eel or bine fish.
3. The spinal cord consists of notochord and fibrous neural tube.
4. Their mouth is marginal and has 4 pairs of gills.
5. They have 5–15 pairs of florets.
6. Their nasal sacs open into the oral cavity.
7. The forebrain forms the brain. Cerebellum absent. There are 10 pairs of carotid nerves.
8. They are known as hagfish.
9. They feed on annelids, molluscs, crustaceans, arthropods or dead fish.
10. The digestive system does not contain a stomach and the intestinal spiral is absent.
11. They are all marine.
12. Their life cycle does not have a larval stage.
13. They are unisexual animals. They are excreted. Eggs are yolky and large.
[KEY WORDS TO REMEMBER CHARACTERISTICS: bine, mouth margin, larynx, larynx, nasal sac, hagfish]
Hag fish
Fish with ugly nature like Bine fish are called hag fish. Their bodies are slender, long and covered with slippery skin. They belong to the Mixini class. For example – Myxine glutinosa.
Petromyzontida
The word Petromyzontida is formed from the Greek words petros meaning stone and myzon meaning sucking. They are known as lamprey. Lives in seas, estuaries and freshwater. Adheres to dead or decaying matter. The larynx creates a suction pool that pulls the rotten material into the mouth. Adults may suck the blood of live fish. Their number of species is 41.
Petromyzontida, Characteristics of Myzontida
1. Their bodies are slender, elongated and without paired fins. 13-100 cm long.
2. They look like eel or bine fish.
3. The body is strong in nature and free of scales.
4. They have a notochord and neural tube in their spinal cord.
5. The mouth has suction cups and keratinized (horned) teeth.
6. They have seven pairs of florets.
7. The nasal sac does not open into the oral cavity.
8. Although marine, they come to freshwater to spawn and die within days of hatching.
9. They are known as lampreys.
10. The digestive system does not have a stomach and the intestine has spiral membranes. 11. Excretion occurs. The life cycle of the amocete larvae is as follows.
12. Joint appendages and exoskeleton are absent. There are dorsal fins and caudal fins.
[KEY WORDS TO REMEMBER CHARACTERISTICS: bine, scaleless, chub, keratin, floret, lamprey, amocite]
Examples: Petromyzon (Petromyzon marinus), Lampetra (Lampetra tridentatus).