In open vascular system, the circulating fluid is known as

Options

A. Blood
B. Perilymph
C. Haemolymph
D. Lymphatic fluid

Answer

To answer the question "In open vascular system, the circulating fluid is known as," we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Open Vascular System**:
- An open vascular system, also known as an open circulatory system, is a type of circulatory system where the blood is not always contained within blood vessels. Instead, it flows freely through body cavities.

**Hint**: Remember that in an open circulatory system, blood is not confined to vessels but instead fills body spaces.

2. **Identify the Circulating Fluid**:
- In organisms with an open vascular system, the circulating fluid is not referred to as blood in the traditional sense. Instead, it is a fluid that fills the sinuses and spaces in the body.

**Hint**: Think about what fluid fills the body cavities in organisms with an open circulatory system.

3. **Recognize the Characteristics of the Fluid**:
- The fluid in an open vascular system lacks respiratory pigments, which are proteins that carry oxygen. Because of this absence, the fluid is often colorless.

**Hint**: Consider what happens to the color of a fluid when it lacks respiratory pigments.

4. **Name the Fluid**:
- The specific term for the circulating fluid in an open vascular system is "hemolymph." This term is used because it combines aspects of blood and lymph, serving similar functions in these organisms.

**Hint**: Recall that the term "hemolymph" is used in the context of invertebrates, particularly arthropods and some mollusks.

5. **Final Answer**:
- Thus, in an open vascular system, the circulating fluid is known as **hemolymph**.

### Summary:
In summary, the circulating fluid in an open vascular system is known as hemolymph, which is colorless due to the absence of respiratory pigments.