Which of the following during respiration obtain water dissolved oxygen by diffusion through their body surface
(a) Cnidarians
(b) Fishes
(c) Amphibians
(d) Reptiles

Options

A. Cnidarians
B. Fishes
C. Amphibians
D. Reptiles.

Answer

To solve the question of which organism obtains dissolved oxygen from water through diffusion across their body surface, we can analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding the Process of Diffusion**:
- Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. For respiration, organisms need a thin body surface to allow for efficient gas exchange.

2. **Analyzing the Options**:
- **(a) Cnidarians**: Cnidarians, such as jellyfish, have a very thin body surface which allows for the diffusion of dissolved oxygen from water directly into their bodies. They can effectively exchange gases through their body surface.
- **(b) Fishes**: Fishes have gills that are specialized for extracting oxygen from water. They do not rely on diffusion through their body surface for respiration.
- **(c) Amphibians**: While amphibians can absorb some oxygen through their skin, they primarily use lungs for respiration as adults. Their skin is not thin enough to rely solely on diffusion for oxygen uptake.
- **(d) Reptiles**: Reptiles have a thick skin that is not permeable enough for effective gas exchange through diffusion. They primarily use lungs for respiration.

3. **Conclusion**:
- Based on the analysis, the only organism that obtains dissolved oxygen from water through diffusion across their body surface is **(a) Cnidarians**.

### Final Answer:
The correct option is **(a) Cnidarians**.