The coelom in arthropods is reduced and known as 'haemocoel' because

Options

A. coelomic fluid flows in the blood vessel
B. blood flows in the blood vessels present in the coelom
C. coelom is filled with haemoglobin
D. blood flows in the coelom.

Explanation

**Step-by-Step Solution:**

1. **Understanding Coelom**: The coelom is a fluid-filled body cavity that is lined by mesoderm. It is an important characteristic for classifying animals. In coelomates, the body cavity is completely lined by mesoderm.

2. **Coelom in Arthropods**: Arthropods are classified as coelomates, meaning they have a coelom. However, in arthropods, this coelom is significantly reduced in size.

3. **Definition of Haemocoel**: The reduced coelom in arthropods is referred to as 'haemocoel'. This term indicates that the body cavity is filled with blood-like fluid rather than being a distinct coelomic cavity.

4. **Function of Haemocoel**: In arthropods, the haemocoel serves as a space where the blood (hemolymph) can flow freely. Unlike in other animals where blood is confined to blood vessels, in arthropods, the blood flows through the haemocoel, bathing the organs directly.

5. **Composition of Hemolymph**: The fluid present in the haemocoel is known as hemolymph, which is typically colorless. Hemolymph serves multiple functions, including nutrient transport, waste removal, and immune responses.

6. **Conclusion**: The reason the coelom in arthropods is reduced and known as haemocoel is that blood flows freely within this cavity instead of being contained within blood vessels. This adaptation is significant for the physiology of arthropods.