The smallest longitudinal blood vessel in Earthworm is

Options

A. dorsal blood vessel
B. ventral blood vessel
C. sub-neural blood vessel
D. supra-oesophageal blood vessel.

Answer

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the smallest longitudinal blood vessel in the earthworm.

2. **Identify the Type of Circulatory System**: Earthworms have a closed circulatory system, meaning that blood is contained within vessels and does not fill body cavities.

3. **Recognize the Major Blood Vessels**: The earthworm has well-developed blood vessels, including the dorsal and ventral blood vessels. These are the main blood vessels in the earthworm.

4. **Locate the Dorsal and Ventral Blood Vessels**: The dorsal blood vessel is located on the upper side of the earthworm and is characterized by a dark mid-dorsal line. The ventral blood vessel is located on the lower side.

5. **Consider Other Blood Vessels**: Other blood vessels include the subneural blood vessel and the supraesophageal blood vessel.

6. **Determine the Smallest Blood Vessel**: Among the options, the supraesophageal blood vessel is identified as the smallest longitudinal blood vessel in the earthworm.

7. **Conclude the Answer**: Therefore, the correct answer is the supraesophageal blood vessel.

### Final Answer:
The smallest longitudinal blood vessel in the earthworm is the **supraesophageal blood vessel**.

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