Which of the following vertebrate organ systems does not open directly to the external environment?

Options

A. Digestive system
B. Circulatory system
C. Excretory system
D. Respiratory system

Explanation

To determine which vertebrate organ system does not open directly to the external environment, we need to analyze each of the organ systems mentioned in the question.

### Step-by-Step Solution:

1. **Identify the Organ Systems**: The main vertebrate organ systems to consider are the digestive system, excretory system, respiratory system, and circulatory system.

2. **Analyze the Digestive System**:
- The digestive system starts at the mouth and includes the esophagus, stomach, and intestines.
- It opens to the external environment at the mouth and the anus, allowing for the intake and expulsion of food.
- **Conclusion**: The digestive system does open to the external environment.

3. **Analyze the Excretory System**:
- The excretory system is responsible for the removal of waste products from the body, primarily through urine.
- It includes organs like the kidneys, ureters, bladder, and urethra.
- The urethra opens to the external environment, allowing for the excretion of urine.
- **Conclusion**: The excretory system does open to the external environment.

4. **Analyze the Respiratory System**:
- The respiratory system is involved in the exchange of gases and includes the nose, trachea, bronchi, and lungs.
- It opens to the external environment through the nose and mouth for the intake of air.
- **Conclusion**: The respiratory system does open to the external environment.

5. **Analyze the Circulatory System**:
- The circulatory system consists of the heart, blood vessels, and blood.
- It is responsible for transporting nutrients, gases, and hormones throughout the body.
- This system does not have any openings to the external environment; all processes occur internally.
- **Conclusion**: The circulatory system does not open to the external environment.

6. **Final Decision**: Based on the analysis, the organ system that does not open directly to the external environment is the **circulatory system**.

### Answer:
The circulatory system is the vertebrate organ system that does not open directly to the external environment.

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