From the following, select the feature that is common to all respiratory systems in animals.

Options

A. Active transport removes carbon dioxide from the respiratory structures .
B. Materials flow in one direction only .
C. An intake system is comprised of a series of tubes.
D. Gases diffuse across a moist membrane.

Answer

To solve the question regarding the common feature of respiratory systems in animals, we can analyze each option provided:

1. **Active transport removes carbon dioxide from the respiratory structures**: This feature is specific to certain lower organisms, like bacteria, and is not a common characteristic of all respiratory systems in animals. Higher organisms do not primarily rely on active transport for gas exchange.

**Hint**: Consider the types of organisms that utilize active transport and whether this applies broadly to all animals.

2. **Material flow in one direction only**: This feature is characteristic of the respiratory system in birds, where air flows in a unidirectional manner through their air sacs and lungs. However, this is not a feature found in all animals, as many animals have bidirectional flow (like mammals).

**Hint**: Think about how different animals breathe and whether they all share the same flow direction for air.

3. **Intake system comprised of a series of tubes**: This feature is specific to insects, which have a tracheal system for gas exchange. It does not apply to all animals, as many have different respiratory structures.

**Hint**: Identify which animals have a tube-like respiratory system and if this is universal across all animal species.

4. **Gases diffuse across a moist membrane**: This is a fundamental process in respiration for many animals, including mammals (through alveoli), amphibians (through skin), and other organisms. The diffusion of gases across a moist membrane is a common feature that facilitates gas exchange in various respiratory systems.

**Hint**: Consider the basic mechanism of gas exchange and whether it is a universal process in different respiratory systems.

After analyzing all the options, the correct answer is:

**Gases diffuse across a moist membrane.**

This feature is common to all respiratory systems in animals, as it is essential for the exchange of oxygen and carbon dioxide in various forms of respiration.

### Summary of Steps:
1. Evaluate the first option about active transport and its applicability to all animals.
2. Analyze the second option regarding unidirectional flow and its limitation to birds.
3. Consider the third option about the tube system and its restriction to insects.
4. Conclude with the fourth option about gas diffusion, recognizing its universal presence in animal respiration.