The word Chondrichthyes is formed from the Greek word chondros meaning cartilage and ichthys meaning fish. Cartilaginous fishes belong to the class Chondrichthyes. They are marine and carnivorous animals. Their number of species is 1,207.

The word Agnatha is formed from the Greek word a meaning jaw and gnathos meaning jaw. Agnatha means jawless. The word Cyclostomata is formed from the Greek word cyclos meaning round and stoma meaning mouth. Vertebrates that lack jaws, scales and jointed appendages are called Cyclostomata. Their endoskeleton is cartilaginous and has 5-15 pairs of gills. They are adapted to a parasitic or carnivorous diet. The living Hagfish, Lamprey and the extinct Ostracoderm) include Agnatha. Agnatha consists of 2 categories. Mixini and Petromyzontida.

The word Chordata is formed from the Latin word chorda meaning chord or notocord or string and ata meaning to bear. Organisms that have rodlike and elastic notochords and hollow nerve cords throughout life or at any stage of life are called chordata. Chordates make up about 3-5% of the world’s population. The number of non- Chordata is 68,500 (Hiclman, 2008). Scientist Balfour (1880) created the order Chordata with animals having notochords. Bateson first named the episode.
Characteristics of chordata
1. Vertebral Cord: Chordates have a notochord in embryo or throughout life. In advanced animals the notochord is replaced by the spinal cord.
2. Brain: Chordates have nerve cords. The anterior end of the nerve cord forms the brain.
3. Symmetry: The posterior part of the nerve cord of Chordata animals forms the symphysis.
4. Pharyngeal gill openings: Chordates have pharyngeal gill openings at any stage of life or throughout life.
5. Heart: Cordata animals have a well-formed heart. The heart is always located at the apex of the intestine.
6. Endostyle: Chordata animals have endostyle. The endostyle later transforms into the thyroid gland.
7. Digestive System: Chordata have complete digestive system. It contains intracellular and extracellular digestive glands.
8. Tail: Cordatas have a post-anal segment elastic tail. Sometimes the tail can get lost during the transition.
9. Circulatory system: Chordates have closed circulatory system. It is made up of blood, blood vessels and blood vessels. It has hepatic portal system.
10. Excretory system: The main excretory system of chordates consists of a pair of kidneys. Through this, nitrogenous wastes are removed from the body.
11. Body skin: The body skin of chordata animals consists of epidermis (derived from ectoderm) and dermis (derived from mesoderm).
12. Endoskeleton: The endoskeleton of chordates is made up of bone or cartilage.
13. Eyes: Cordatas eyes are simple in nature. It originates from the brain.
14. Legs: Vertebrate animals have two pairs of legs.
15. Segmentation: Segmentation of chordates is limited to body wall, brain and tail.
16. Genus: Chordata are unisexual. They have sex. Asexual reproduction is absent.
17. Osmoregulation: Osmoregulation of chordates occurs through the kidneys.
18. Silome: True silome is present in the body of chordates. Surrounding the heart is the pericardial cavity.
[key words to remember features: spinal cord, brain, mesentery, pharyngeal pharynx, endostyle, tail]

1. Vertebral Cord: Chordates have a notochord in embryo or throughout life. In advanced animals the notochord is replaced by the spinal cord.
2. Brain: Chordates have nerve cords. The anterior end of the nerve cord forms the brain.
3. Symmetry: The posterior part of the nerve cord of Chordata animals forms the symphysis.
4. Pharyngeal gill openings: Chordates have pharyngeal gill openings at any stage of life or throughout life.
5. Heart: Cordata animals have a well-formed heart. The heart is always located at the apex of the intestine.
6. Endostyle: Chordata animals have endostyle. The endostyle later transforms into the thyroid gland.
7. Digestive System: Chordata have complete digestive system. It contains intracellular and extracellular digestive glands.
8. Tail: Cordatas have a post-anal segment elastic tail. Sometimes the tail can get lost during the transition.
9. Circulatory system: Chordates have closed circulatory system. It is made up of blood, blood vessels and blood vessels. It has hepatic portal system.
10. Excretory system: The main excretory system of chordates consists of a pair of kidneys. Through this, nitrogenous wastes are removed from the body.
11. Body skin: The body skin of chordata animals consists of epidermis (derived from ectoderm) and dermis (derived from mesoderm).
12. Endoskeleton: The endoskeleton of chordates is made up of bone or cartilage.
13. Eyes: Cordatas eyes are simple in nature. It originates from the brain.
14. Legs: Vertebrate animals have two pairs of legs.
15. Segmentation: Segmentation of chordates is limited to body wall, brain and tail.
16. Genus: Chordata are unisexual. They have sex. Asexual reproduction is absent.
17. Osmoregulation: Osmoregulation of chordates occurs through the kidneys.
18. Silome: True silome is present in the body of chordates. Surrounding the heart is the pericardial cavity.
[key words to remember features: spinal cord, brain, mesentery, pharyngeal pharynx, endostyle, tail]

The word Chordata is formed from the Latin word chorda meaning chord or notocord or string and ata meaning to bear. Organisms that have rodlike and elastic notochords and hollow nerve cords throughout life or at any stage of life are called chordata. Chordates make up about 3-5% of the world’s population. The number of non- Chordata is 68,500 (Hiclman, 2008). Scientist Balfour (1880) created the order Chordata with animals having notochords. Bateson first named the episode.

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).

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).

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).

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.

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.