Class Chondrichthyes
காண்ட்ரிக்தீஸ் வகுப்பு
Stop confusing Chondrichthyes with Osteichthyes. The absence of an operculum and the presence of placoid scales are your primary diagnostic markers. If you miss the ventral mouth position, you lose the mark.
The idea
Chondrichthyes are marine cartilaginous fishes with a persistent notochord and a streamlined body. They possess placoid scales and a ventral mouth. Crucially, they lack an air bladder, necessitating constant swimming to prevent sinking. Fertilization is internal, and many are viviparous.
Why it matters
Cartilaginous skeletons are lighter than bone, aiding in buoyancy, while the absence of an air bladder necessitates constant movement for gas exchange and depth control.
NEET lens
NEET focuses on the comparison between Chondrichthyes and Osteichthyes. Memorize the differences: placoid vs cycloid scales, ventral vs terminal mouth, and presence vs absence of operculum. Expect at least one matching-type question on these features. Do not ignore the 'constant swimming' requirement; it is a favorite for assertion-reasoning questions.
Clinical significance
The lack of an air bladder in these fishes demonstrates the evolutionary necessity of buoyancy control. In humans, the failure of the Eustachian tube to regulate middle ear pressure mirrors the concept of hydrostatic balance. Understanding these physiological adaptations is fundamental to comparative anatomy.
Introduction
Sharks have been patrolling the oceans for over 400 million years — virtually unchanged. Class Chondrichthyes, the cartilaginous fish, are among the most ancient vertebrates. Their skeleton is made entirely of cartilage, they have no swim bladder, and their skin is covered in tiny tooth-like scales. Understanding how they differ from bony fish is the core of this NEET topic.
Core concept
Class Chondrichthyes (cartilaginous fish) features: (1) endoskeleton made of cartilage — no bone; (2) body covered with placoid scales (tooth-like, also called dermal denticles); (3) no operculum — gill slits are exposed (5–7 pairs visible on surface); (4) no swim bladder — must swim continuously or sink (or have an oil-filled liver for buoyancy); (5) cold-blooded (poikilothermous); (6) mouth on the ventral surface; (7) internal fertilisation; (8) mostly viviparous; (9) notochord persistent throughout life (though modified). Examples: Scoliodon (dogfish shark), Pristis (saw fish), Torpedo (electric ray — produces electric current for stunning prey), Trygon (sting ray), Carcharodon (great white shark).
Go deeper
Torpedo (electric ray) produces electric discharges from modified muscle cells (electrocytes/electroplaques) — used to stun prey and for defence. The absence of a swim bladder means sharks must keep swimming to avoid sinking, or rely on their large, oil-rich liver (squalene) for buoyancy. Placoid scales are structurally identical to vertebrate teeth — they have an enamel-like coating and a dentine core.
From Class 10
In Class 10, you learned about sharks as cartilaginous fish. Class Chondrichthyes is tested at NEET level on the features that contrast with bony fish — especially the absence of an operculum and swim bladder, and the presence of placoid scales.