Respiratiry organs in aquatic arthropods like cray
fish, prawn and molluscs like Unio are

Options

A. Body wall
B. Lungs
C. Tracheae
D. Gills

Answer

To answer the question regarding the respiratory organs in aquatic arthropods like crayfish, prawn, and mollusks like Unio, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Organisms**: The question mentions aquatic arthropods such as crayfish and prawns, as well as mollusks like Unio. It is important to recognize that both groups are aquatic.

2. **Understand Respiratory Organs**: Different organisms have specialized structures for respiration. In general, respiratory organs are involved in the exchange of gases (oxygen and carbon dioxide).

3. **Differentiate Between Terrestrial and Aquatic Arthropods**:
- Terrestrial arthropods (like insects) primarily use tracheae for gas exchange.
- Aquatic arthropods (like crayfish and prawns) have adapted to their environment and utilize gills for respiration.

4. **Identify the Correct Respiratory Organ**:
- For aquatic arthropods, the primary respiratory organs are gills. Gills are specialized structures that allow for the efficient exchange of gases in water.

5. **Consider Mollusks**:
- Similar to aquatic arthropods, many aquatic mollusks, including Unio, also possess gills for respiration.

6. **Evaluate the Options**:
- The options provided are body wall, lungs, tracheae, and gills.
- Body wall is incorrect as it is not a specialized organ for gas exchange in these organisms.
- Lungs are not applicable here as they are found in terrestrial vertebrates.
- Tracheae are not suitable for aquatic organisms since they are designed for air-breathing.
- Gills are the correct answer as they are the specialized respiratory organs for both aquatic arthropods and certain mollusks.

### Conclusion:
The correct answer is **gills**.