Class Cyclostomata: Jawless Fishes Lamprey and Hagfish

Class Cyclostomata contains the jawless fishes — the lampreys and hagfish. The name means "round mouth," describing their circular, jawless, sucking mouth. They are eel-like in shape but are not true eels; they represent the only living agnathans (jawless vertebrates), with a cranium and notochord but no vertebrae or jaws.

They matter because they are living fossils of the earliest vertebrates. Lampreys parasitise other fishes, while hagfish scavenge on the ocean floor. Their simple body plan — no scales, no paired fins, no stomach in some — shows what vertebrates looked like before jaws, limbs and complex digestive systems evolved.

Key Concepts

• Circular sucking mouth: jawless, ringed with horny teeth; a rasping tongue bores into prey.
• Cartilaginous cranium: the brain is protected by cartilage, but there are no vertebrae — the notochord persists.
• Gill slits: lampreys have 7 pairs of gill slits; hagfish have 5–15 pairs, opening into a common chamber.
• No scales or paired fins: the skin is smooth and slimy; only median fins are present.
• Single median nostril: one nostril on top of the head, unlike the paired nostrils of jawed fishes.
• Ammocoete larva: lamprey larvae are blind filter feeders that live buried in stream mud for years before metamorphosis.

How It Works

1. An adult sea lamprey attaches its sucker-like mouth to the side of a fish.
2. The horny teeth grip the skin while the rasping tongue scrapes a hole through it.
3. The lamprey secretes an anticoagulant so the wound keeps bleeding, and feeds on blood and tissue fluids.
4. After feeding for weeks, it detaches and migrates up freshwater rivers to spawn.
5. Adults die after spawning; eggs hatch into ammocoete larvae that filter feed for 3–7 years before transforming into parasitic adults.

Worked Example

A student examines a lamprey and counts 7 pairs of gill slits. How many individual gill openings does it have?

1. Pairs of gill slits = 7
2. Sides of the body = 2 (left + right)
3. Total gill openings = 7 x 2 = 14

These 14 openings each lead to a gill pouch. Hagfish differ by having their slits open into fewer external pores, but the paired arrangement is the same plan.

Common Misconceptions

• "Lampreys are eels." True eels are bony jawed fishes; lampreys belong to a far older, jawless lineage.
• "Hagfish have backbones." They have a cranium but no vertebrae — the notochord supports the body throughout life.
• "All jawless fishes are extinct." Lampreys and hagfish are alive today and ecologically successful.

Real-World Applications

• Sea lamprey population control in the North American Great Lakes protects a billion-dollar fishery.
• Hagfish slime proteins are researched for ultra-strong fibres and biodegradable materials.
• Lampreys are used in neuroscience as a simple model of the vertebrate spinal cord and locomotion.

Cyclostomes are jawless, scaleless fishes with round sucking mouths, persistent notochords and no paired fins. As the only living agnathans, lampreys and hagfish preserve the body plan of the first vertebrates.