The bacteria present in the body surface of earthworm are destroyed by the coelomic fluid.

The coelomic fluid is a liquid found in the coelom, which is the body cavity of earthworms. It plays a crucial role in various physiological processes, including defense against pathogens. Here's how the coelomic fluid helps in destroying bacteria:

1. Antibacterial properties:
The coelomic fluid contains various substances that have antibacterial properties. These substances can directly kill or inhibit the growth of bacteria. For example, lysozyme is an enzyme present in the coelomic fluid that can break down the cell walls of bacteria, leading to their destruction. Other antimicrobial peptides and proteins present in the fluid can also have similar bactericidal effects.

2. Phagocytosis:
The coelomic fluid also contains specialized cells called coelomocytes, which play a crucial role in the immune response of earthworms. These coelomocytes have the ability to recognize and engulf bacteria through a process called phagocytosis. Once engulfed, the bacteria are destroyed by the enzymes and toxic substances present within the coelomocytes.

3. Formation of barriers:
The coelomic fluid can create physical barriers that prevent the colonization and spread of bacteria on the body surface of the earthworm. For example, the fluid can form a protective film over the body surface, making it difficult for bacteria to attach and multiply. Additionally, the fluid can flush out bacteria from the body surface through peristaltic movements, thereby reducing their population.

4. Activation of immune responses:
The coelomic fluid contains immune cells that can recognize and respond to bacterial infections. These immune cells can release chemical signals, such as cytokines, to attract more immune cells to the site of infection. This immune response helps in the destruction of bacteria and the resolution of the infection.

In summary, the coelomic fluid of earthworms plays a vital role in the defense against bacteria present on their body surface. It contains antibacterial substances, facilitates phagocytosis, forms physical barriers, and activates immune responses to destroy and eliminate bacteria. Therefore, the correct answer is option 'B' - By Coelomic fluid.