Class Cyclostomata
சைக்ளோஸ்டோமேட்டா வகுப்பு
Cyclostomata are the most primitive vertebrates. They lack jaws, yet they dominate as ectoparasites on fish. Do not confuse their cartilaginous cranium with the Chondrichthyes; their evolutionary path is distinct.
The idea
Cyclostomes possess an elongated body with 6-15 pairs of gill slits for respiration. They lack jaws and have a cartilaginous cranium and vertebral column. Their skin is devoid of scales and paired fins. They are anadromous, migrating to fresh water for spawning, followed by death.
Why it matters
This specialized life cycle and jawless structure allow them to exploit specific niches as parasites on larger fish hosts.
NEET lens
NEET focuses on the 'anadromous' migration pattern and the number of gill slits. Remember: Cyclostomes die after spawning. Do not mistake them for bony fish; they lack scales entirely.
Clinical significance
In the ecosystem, Petromyzon acts as a biological control agent by regulating fish populations. Their parasitic feeding mechanism provides a model for studying host-pathogen interactions and nutrient extraction efficiency in aquatic biology.
Introduction
Before jaws evolved, all vertebrates were jawless filter feeders. Most of them are extinct — but two survivors remain: lampreys and hagfish, belonging to Class Cyclostomata. These eel-like animals have a circular, sucker-like mouth with no jaws, no paired fins, and a notochord that persists throughout life. They are the most primitive living vertebrates and a crucial link in understanding vertebrate evolution.
Core concept
Class Cyclostomata features: (1) jawless vertebrates (Agnatha); (2) circular, sucker-like mouth — hence 'cyclostomata' (cyclo = circular, stoma = mouth); (3) no paired fins; (4) notochord persists throughout life (not replaced by a true vertebral column); (5) parasitic on other fish; (6) body elongated, eel-like; (7) 7 pairs of gill pouches; (8) marine but migrate to freshwater to breed; (9) after spawning, they die (semelparous); (10) larvae (called ammocoetes in lamprey) are filter feeders. Two examples: Petromyzon (lamprey) and Myxine (hagfish).
Go deeper
The ammocoete larva of lamprey spends years buried in sediment as a filter feeder before metamorphosing into the parasitic adult — this larval stage is considered a model for ancestral chordate feeding. Hagfish are notorious for producing large quantities of slime as a defence mechanism. Both lampreys and hagfish can tie themselves in knots — used to generate leverage when tearing flesh from a host fish.
From Class 10
You may not have encountered Cyclostomata in Class 10. This is a Class Cyclostomata — the most primitive living vertebrates — that sits between invertebrates and jawed fish. At NEET level, know their key features: jawless, no paired fins, parasitic.