In Amphioxus, excretion takes place through protonephridia. Protonephridia are simple excretory structures found in invertebrates, including Amphioxus. These structures help in the removal of metabolic waste products from the body.

Protonephridia: The primary excretory organs in Amphioxus are protonephridia. They are also known as flame cells due to their shape. Each protonephridium consists of a tube-like structure with ciliated cells called flame cells. These flame cells are responsible for the filtration and excretion of waste materials.

Filtration: The process of excretion in Amphioxus begins with the filtration of body fluids. The flame cells have specialized structures called cilia that create a current of fluid within the protonephridium. This fluid contains waste materials such as ammonia, urea, and other metabolic waste products.

Selective reabsorption: As the fluid passes through the protonephridium, selective reabsorption takes place. Some essential substances, such as water and ions, are reabsorbed back into the body while waste materials remain in the fluid.

Excretion: The final step in the excretory process is the excretion of waste materials. The filtered fluid, now containing waste products, is expelled from the body through a pore called the nephridiopore. This waste material is then released into the external environment.

Advantages of protonephridia: Protonephridia have several advantages in excretion. Firstly, they are simple structures that do not require a complex system of tubules or organs like kidneys in higher vertebrates. This simplicity allows for efficient excretion in Amphioxus. Secondly, the cilia present in the flame cells help in creating a current of fluid, aiding in filtration and movement of waste materials.

Conclusion: In Amphioxus, excretion takes place through protonephridia, which are simple excretory structures consisting of flame cells. These structures filter body fluids, selectively reabsorb essential substances, and excrete waste materials through a nephridiopore. Protonephridia are advantageous in their simplicity and efficiency in excretion in Amphioxus. Therefore, option B, Protonephridium, is the correct answer.