Class Osteichthyes: Bony Fishes and the Air Bladder
Biology · Animal Kingdom · NEET
Class Osteichthyes are the bony fishes. They have a skeleton made of bone, a gill cover (operculum), and an air bladder that helps them float without sinking. Memory hook: "Osteo = bone; bony fish carry an air balloon (air bladder) so they never have to keep swimming."
Key NEET features of a bony fish (Osteichthyes): terminal mouth, four gill pairs under a bony operculum, cycloid/ctenoid scales, a two-chambered heart, and the buoyancy-regulating air bladder that Chondrichthyes lack.
Your doubts, answered
What does Osteichthyes actually mean, in simple words?
'Osteo' means bone. So Osteichthyes = bony fishes. Their inner skeleton (endoskeleton) is made of hard bone, not cartilage. This one word tells you the key feature: bone. Chondrichthyes ('chondro' = cartilage) is the opposite group with a soft cartilage skeleton. For NEET, always link the name to the skeleton type.
Why do bony fishes have an air bladder and cartilaginous fishes don't?
The air bladder (also called swim bladder) is a gas-filled sac that regulates buoyancy. It lets the bony fish stay at any depth without effort, like a built-in float. Cartilaginous fishes (Chondrichthyes) have NO air bladder, so they must swim constantly to avoid sinking. This is a very common NEET match-the-column point: air bladder = Osteichthyes only.
How are the gills different in Osteichthyes vs Chondrichthyes?
Osteichthyes have four pairs of gills covered by a bony flap called the operculum (gill cover). Chondrichthyes have separate gill slits with NO operculum. So 'operculum present' points only to bony fishes. NCERT even uses this in a trap: 'All pisces have gills covered by an operculum' is WRONG, because cartilaginous fishes lack an operculum.
How many heart chambers does a bony fish have?
Osteichthyes have a two-chambered heart: one auricle and one ventricle. This is the same as most fishes. Do not confuse it with amphibians (3-chambered) or birds and mammals (4-chambered). Fishes = 2 chambers is a fixed NEET fact.
Is Rohu (Labeo) a bony fish? What are the standard NCERT examples?
Yes. Labeo (Rohu) is a freshwater bony fish. NCERT examples of Osteichthyes: Marine - Exocoetus (Flying fish), Hippocampus (Sea horse); Freshwater - Labeo (Rohu), Catla (Katla), Clarias (Magur); Aquarium - Betta (Fighting fish), Pterophyllum (Angel fish). Memorise these exact names; NEET repeatedly asks you to sort them from cartilaginous fishes and molluscs.
What kind of scales and mouth position do bony fishes have?
Bony fishes have cycloid or ctenoid scales on the skin, and the mouth is mostly terminal (at the front tip). Compare: Chondrichthyes have placoid scales and a ventral (lower-side) mouth. So 'cycloid/ctenoid scales + terminal mouth' = Osteichthyes.
Are bony fishes cold-blooded, and how do they reproduce?
Yes, they are cold-blooded (poikilothermic). Sexes are separate. Fertilisation is usually external, they are mostly oviparous (egg-laying), and development is direct (no larval stage that looks different from the adult).
⚠️ The NEET trap ✗ All Pisces (fishes) have gills covered by an operculum. ✓ Only Osteichthyes have gills covered by an operculum. Chondrichthyes have separate gill slits with NO operculum. Also, the air bladder belongs only to Osteichthyes. 🧠 Operculum + air bladder = BONY fish only. If a statement says ALL fishes have these, it is a trap.
Real NEET questions
2020
Match the following columns and select the correct option.
Column-I
(a) 6-15 pairs of gill slits
(b) Heterocercal caudal fin
(c) Air bladder
(d) Poison sting
Column-II
(i) Trygon
(ii) Cyclostomes
(iii) Chondrichthyes
(iv) Osteichthyes
A · a-iv, b-ii, c-iii, d-i
B · a-i, b-iv, c-iii, d-ii
C · a-ii, b-iii, c-iv, d-i ✓
D · a-iii, b-iv, c-i, d-ii
Solution: Cyclostomes have 6-15 pairs of gill slits, so (a)-(ii). The unequal-lobed heterocercal caudal fin is a Chondrichthyes feature, so (b)-(iii). The air bladder that regulates buoyancy is present only in bony fishes, so (c)-(iv) Osteichthyes. The poison sting is borne by Trygon (sting ray), so (d)-(i). Correct option is C.
2026
Select the set of fishes which belong to the class Osteichthyes:
A · Devil fish, Cuttlefish and Hagfish
B · Saw fish, Fighting fish and Dog fish
C · Starfish, Hagfish and Cuttlefish
D · Flying fish, Angel fish and Fighting fish ✓
Solution: Osteichthyes are bony fishes. Flying fish (Exocoetus), Angel fish (Pterophyllum) and Fighting fish (Betta) are all NCERT bony-fish examples, so option D is correct. The other options mix in molluscs (Devil fish/Octopus, Cuttlefish), a cyclostome (Hagfish/Myxine), chondrichthyans (Saw fish/Pristis, Dog fish/Scoliodon) and an echinoderm (Starfish).
2016
Choose the correct statement.
A · All mammals are viviparous
B · All cyclostomes do not possess jaws and paired fins ✓
C · All reptiles have a three-chambered heart
D · All pisces have gills covered by an operculum
Solution: Cyclostomes lack jaws and paired fins, so (b) is correct. (a) is wrong because egg-laying mammals exist; (c) is wrong because crocodiles have a four-chambered heart; (d) is wrong because only Osteichthyes have an operculum, while Chondrichthyes have separate gill slits without one.
Solved Animal Kingdom NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the main difference between Osteichthyes and Chondrichthyes?
Osteichthyes have a bony skeleton, an operculum, an air bladder, cycloid/ctenoid scales and a mostly terminal mouth. Chondrichthyes have a cartilaginous skeleton, no operculum, no air bladder, placoid scales and a ventral mouth.
What is the function of the air bladder in Osteichthyes?
The air bladder regulates buoyancy. It is a gas-filled sac that lets the fish float and stay at a chosen depth without constant swimming.
How many gill pairs do bony fishes have?
Bony fishes have four pairs of gills, and these are covered by an operculum on each side.
Are all Osteichthyes marine?
No. Osteichthyes include both marine and freshwater fishes. For example, Exocoetus and Hippocampus are marine, while Labeo, Catla and Clarias are freshwater.
What type of heart do bony fishes have?
A two-chambered heart with one auricle and one ventricle. Bony fishes are also cold-blooded (poikilothermic).
