Air bladder is

Options

A. respiratory organ of insects
B. excretory organ of mammals
C. a cnidarian
D. hydrostatic organ of bony fishes

Answer

### Step-by-Step Solution:

1. **Identify the Organ**: The question asks about the air bladder, which is an organ found in certain animals, particularly fishes.

2. **Classify the Organism**: The air bladder is primarily associated with fishes, which belong to the class Pisces. Pisces is part of the subphylum Vertebrata within the phylum Chordata.

3. **Differentiate Between Fish Types**: The class Pisces can be divided into two main groups:
- **Osteichthyes**: These are bony fishes that have an endoskeleton made of bone.
- **Chondrichthyes**: These are cartilaginous fishes that have an endoskeleton made of cartilage.

4. **Function of the Air Bladder**: The air bladder serves as a hydrostatic organ in bony fishes (Osteichthyes). Its primary function is to help maintain buoyancy, allowing the fish to float and stabilize at various depths in the water.

5. **Conclusion**: Based on the information, the air bladder is best described as a hydrostatic organ of bony fishes. It is not related to respiratory or excretory functions in other organisms like insects or cnidarians.

### Final Answer:
The air bladder is a hydrostatic organ of bony fishes (Osteichthyes).

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