The respiratory organs in a fresh water mussel are

Options

A. pulmonary sac
B. book lungs
C. Gills
D. tracheae

Answer

**Step-by-Step Solution:**

1. **Identify the Organism**: The question refers to a freshwater mussel. An example of a freshwater mussel is the genus *Unionidae*.

2. **Determine the Respiratory Organs**: Freshwater mussels, like many mollusks, primarily use gills for respiration.

3. **Understand the Functions of Gills**:
- **Respiration**: Gills are used to extract oxygen from water, similar to how fish use their gills.
- **Filter Feeding**: Gills also help in feeding by moving food particles towards the mouth.
- **Incubation**: In female mussels, gills serve a reproductive function by incubating baby mussels until they are mature enough to be released.

4. **Evaluate Other Options**:
- **Pulmonary Sac**: This is found in land mollusks and acts as a lung, so it is not applicable here.
- **Book Lungs**: These are respiratory organs found in arachnids like spiders and scorpions, making this option incorrect.
- **Trachea**: This is a respiratory system found in insects and is not relevant to mussels.

5. **Conclude the Correct Answer**: The correct answer is **gills**, as they are the primary respiratory organs in freshwater mussels.

**Final Answer**: The respiratory organs in a freshwater mussel are **gills**.

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