Choose the correct statements :
I. Cartilaginous fish swims continously due to the presence of an air bladder.
II. Osteichthyes have four pairs of gills that are covered by an operculum.
III. The notochord is persistent throughout life in Chondrichthyes.
IV. Teeth of cartilaginous fishes are modified placoid scales.

Options

A. Only (I) & (II) are correct
B. Only (II), (III) & (IV) are correct
C. Only (II) & (IV) are correct
D. All are correct

Answer

To solve the question, we need to analyze each statement provided about cartilaginous fish (Chondrichthyes) and bony fish (Osteichthyes) to determine which ones are correct.

### Step-by-Step Solution:

1. **Statement I: Cartilaginous fish swims continuously due to the presence of an air bladder.**
- **Analysis:** This statement is incorrect. Cartilaginous fish, such as sharks and rays, do not possess an air bladder. Instead, they have a large liver filled with oil that helps them maintain buoyancy. They swim continuously to avoid sinking due to their denser bodies compared to water.
- **Conclusion:** **Incorrect**

2. **Statement II: Osteichthyes have four pairs of gills that are covered by an operculum.**
- **Analysis:** This statement is correct. Osteichthyes, or bony fish, typically have four pairs of gills that are covered by a bony flap known as the operculum. This structure helps in respiration and protects the gills.
- **Conclusion:** **Correct**

3. **Statement III: The notochord is persistent throughout life in Chondrichthyes.**
- **Analysis:** This statement is correct. In Chondrichthyes, the notochord is retained throughout their life, providing structural support, unlike in bony fish where it is replaced by the vertebral column during development.
- **Conclusion:** **Correct**

4. **Statement IV: Teeth of cartilaginous fishes are modified placoid scales.**
- **Analysis:** This statement is correct. The teeth of cartilaginous fishes are indeed modified placoid scales, which are small, tooth-like structures that provide protection and aid in feeding.
- **Conclusion:** **Correct**

### Final Evaluation:
- **Correct Statements:** II, III, and IV
- **Incorrect Statement:** I

### Answer:
The correct statements are II, III, and IV.

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