Give the role of protonephridia.

Explanation

### Step-by-Step Solution:

1. **Definition of Protonephridia**: Protonephridia are excretory structures found in certain invertebrates, primarily in flatworms. They serve a similar function to nephrons in higher organisms.

2. **Function of Protonephridia**: The primary role of protonephridia is to facilitate excretion. They perform ultrafiltration, which is the process of filtering waste products from the body fluids.

3. **Reabsorption Process**: In addition to ultrafiltration, protonephridia also participate in the reabsorption of essential substances. This means that after filtering the waste, they can reclaim useful compounds that the organism needs to retain.

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