Flow of water and blood to the respiratory organs is countercurrent in these animals

Options

A. Sponges, coelenterates, flatworms
B. Pisces
C. Mammals
D. Aquatic arthropods & molluscs

Answer

**Step-by-Step Solution:**

1. **Understand the Question**: The question asks which animals exhibit a countercurrent flow of water and blood to their respiratory organs.

2. **Identify the Options**: The options provided are:
- A) Sponges, coelenterates, and flatworms
- B) Pisces (fish)
- C) Mammals
- D) Aquatic earthworms and mollusks

3. **Analyze Each Option**:
- **Option A**: Sponges, coelenterates, and flatworms do not have specialized respiratory organs like gills or lungs; they rely on diffusion for gas exchange.
- **Option B**: Pisces (fish) possess gills, which are specialized organs for respiration. In fish, the flow of water over the gills is in the opposite direction to the flow of blood within the gills, creating a countercurrent exchange system.
- **Option C**: Mammals have lungs and do not exhibit a countercurrent exchange system in their respiratory process.
- **Option D**: Aquatic earthworms and mollusks have different mechanisms for respiration and do not utilize a countercurrent system like fish.

4. **Conclude the Answer**: Based on the analysis, the only group that exhibits a countercurrent flow of water and blood to their respiratory organs is **B) Pisces (fish)**.

5. **Final Answer**: The correct answer is **Pisces**.

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