Which organ system is lacking in the animals that bear water vascular system?

Options

A. Nervous system
B. Excretory system
C. Circulatory system
D. Sensory system

Explanation

To answer the question about which organ system is lacking in animals that bear a water vascular system, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Organ Systems in Question**: The options given are the nervous system, excretory system, circulatory system, and sensory system.

2. **Understand the Water Vascular System**: The water vascular system is a unique feature found in certain invertebrates, particularly echinoderms (like starfish and sea urchins). It is primarily used for locomotion, feeding, and respiration.

3. **Examine the Role of Each Organ System**:
- **Nervous System**: Echinoderms do have a simple nervous system, which allows them to respond to environmental stimuli.
- **Excretory System**: Echinoderms do not have a specialized excretory system. They rely on the water vascular system and diffusion through their body surface for waste removal.
- **Circulatory System**: Echinoderms have a form of an open circulatory system, but it is not as complex as in vertebrates.
- **Sensory System**: They possess a basic sensory system to detect light and touch.

4. **Conclusion**: Among the options provided, the excretory system is the one that is lacking in animals that bear a water vascular system. They do not require a specialized excretory system because they can perform excretion through the water current and diffusion.

### Final Answer:
The organ system that is lacking in animals that bear a water vascular system is the **excretory system**.

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