Fishes and amphibians

Class Cyclostomata: no jaws

The cyclostomes are the only living vertebrates without jaws. Their mouth is round and works as a sucker. Many of them live as ectoparasites on fishes: a lamprey clings to a fish with its mouth and feeds on its blood.

• The body is long and thin, like an eel.
• There are 6 to 15 pairs of gill slits for breathing.
• The skin has no scales, and there are no paired fins.
• The cranium and the vertebral column are made of cartilage.
• The circulation is closed.

Cyclostomes live in the sea, but they swim up into fresh water to spawn. The adults die within a few days of spawning. Their larvae change form and return to the ocean.

Examples: Petromyzon, the lamprey, and Myxine, the hagfish.

Class Chondrichthyes: skeletons of cartilage

These are the sharks and rays. They live in the sea, and their bodies are streamlined.

• The endoskeleton is made of cartilage, not bone.
• The mouth is on the lower side of the head.
• The notochord persists throughout life.
• The gill slits open separately. There is no operculum, no gill cover.
• The skin is tough and carries tiny placoid scales. The teeth are modified placoid scales, and they point backwards.
• The jaws are very powerful. These fishes are predators.
• There is no air bladder. So they must keep swimming, or they sink.
• The heart has two chambers, one atrium and one ventricle.

Some have special weapons. Torpedo, the electric ray, has electric organs that can stun prey. Trygon, the stingray, has a poison sting.

They are cold-blooded, or poikilothermous: their body temperature changes with their surroundings. The sexes are separate. In males, the pelvic fins carry claspers, which pass sperm into the female. So fertilisation is internal, and many of these fishes are viviparous, giving birth to live young.

Examples: Scoliodon, the dogfish, Pristis, the sawfish, Carcharodon, the great white shark, and Trygon, the stingray.

Class Osteichthyes: skeletons of bone

The bony fishes live in both the sea and fresh water.

• The endoskeleton is made of bone.
• The body is streamlined, and the mouth is mostly at the front.
• There are four pairs of gills, covered on each side by a flap, the operculum.
• The skin carries cycloid or ctenoid scales.
• An air bladder lets the fish float at any depth without swimming.
• The heart has two chambers, and the fish is cold-blooded.

The sexes are separate. Fertilisation is usually external, in the water. Most bony fishes are oviparous: they lay eggs. Development is direct, so the young look like small adults.

Examples from the sea: Exocoetus, the flying fish, and Hippocampus, the sea horse. From fresh water: Labeo, the rohu, Catla, the katla, and Clarias, the magur. In an aquarium: Betta, the fighting fish, and Pterophyllum, the angel fish.

The two classes of fishes, side by side

The sharks and rays form Chondrichthyes, and the bony fishes form Osteichthyes.

Class Amphibia: two lives

The name means a double life. Amphibians live both in water and on land.

• Most have two pairs of limbs. The body is divided into a head and a trunk, and some have a tail.
• The skin is moist and has no scales.
• The eyes have eyelids. A tympanum, a drum-like membrane, serves as the ear.
• The gut, the urinary ducts and the reproductive ducts all open into one chamber, the cloaca, which opens to the outside.
• They breathe with gills, lungs and through the skin.
• The heart has three chambers: two atria and one ventricle.
• They are cold-blooded.

The sexes are separate. Fertilisation is external, and amphibians are oviparous. Development is indirect: a frog's egg hatches into a tadpole, which breathes with gills before it changes into a frog.

Examples: Bufo, the toad, Rana, the frog, Hyla, the tree frog, Salamandra, the salamander, and Ichthyophis, a limbless amphibian that lives in damp soil.

Gill slits and the gill cover

Move the slider through the three classes of fishes. Count the openings on one side of the head.

Try it in Python

The program is a small key. It asks four questions about an animal and names its class.

Practise with new numbers

Common mistakes

• "A shark is a bony fish." Sharks and rays have a skeleton of cartilage. They belong to Chondrichthyes.
• "All fishes float easily." Cartilaginous fishes have no air bladder, so they must keep swimming.
• "Every fish has an operculum." Only bony fishes do. A shark's gill slits open separately.
• "Amphibians have dry, scaly skin." That is a reptile. Amphibian skin is moist and has no scales.
• "Ichthyophis is a snake." It is a limbless amphibian, with moist skin and a life in damp soil.

Remember these

• Cyclostomata: no jaws, a sucking mouth, 6 to 15 pairs of gill slits, no scales and no paired fins. Petromyzon, Myxine.
• Chondrichthyes: cartilage skeleton, ventral mouth, separate gill slits, placoid scales, no air bladder, claspers and internal fertilisation.
• Osteichthyes: bony skeleton, four pairs of gills under an operculum, an air bladder, usually external fertilisation.
• Fishes have a two-chambered heart, and all of them are cold-blooded.
• Amphibia: moist skin without scales, a tympanum, a cloaca, a three-chambered heart, and a tadpole stage.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How does a lamprey feed, and which missing part explains its round mouth?

2. Give two ways in which a rohu differs from a dogfish.

3. What are claspers, and what do they tell you about fertilisation?

4. Name three ways in which a frog takes in oxygen.

5. What is a cloaca?