Which one is not the charater of Osteichthyes?

Options

A. Terminal mouth
B. Gills with operculum
C. Body covered by placoid scales
D. Air bladder regulate buoyancy

Answer

To determine which characteristic is not associated with Osteichthyes (bony fishes), we can analyze the provided options step by step.

### Step-by-Step Solution:

1. **Understand Osteichthyes**: Osteichthyes, commonly known as bony fishes, are characterized by having a bony skeleton instead of a cartilaginous one. This is a fundamental feature that distinguishes them from other fish groups like Chondrichthyes (cartilaginous fishes).

2. **Evaluate the Options**:
- **Option 1**: Terminal mouth - This is a characteristic of many bony fishes, as they often have a mouth located at the end of their bodies.
- **Option 2**: Gills with operculum - Bony fishes have gills covered by an operculum, which is a bony flap that protects the gills. This is indeed a characteristic of Osteichthyes.
- **Option 3**: Body covered by placoid scales - Placoid scales are characteristic of Chondrichthyes (like sharks) and not Osteichthyes. Bony fishes typically have either cycloid or ctenoid scales.
- **Option 4**: Air bladder regulates buoyancy - Bony fishes possess an air bladder (swim bladder) that helps them maintain buoyancy in water, which is a defining feature of Osteichthyes.

3. **Identify the Incorrect Characteristic**: Based on the evaluation, option 3 (body covered by placoid scales) is not a characteristic of Osteichthyes, as they do not have placoid scales.

### Conclusion:
The characteristic that is not associated with Osteichthyes is **Option 3: Body covered by placoid scales**.

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