Excretion in Hemichordates occurs through the use of a structure called the glomerulus. This is a small tuft of capillaries located within the body cavity of the organism. The glomerulus acts as a filtration system, removing waste products and excess water from the circulatory system.

Option 2, the pronephron, is a type of excretory structure found in vertebrates, not hemichordates. It is an embryonic kidney that eventually develops into more complex renal structures.

Option 3, the mesonephron, is also an excretory structure found in vertebrates, specifically in certain groups of fish and amphibians. It functions as the main kidney in these organisms and is responsible for filtering waste from the blood.

Option 4, the metanephron, is the final stage of kidney development in vertebrates, where the pronephric and mesonephric structures have been replaced by a more advanced kidney.

Therefore, the correct answer is option 1, the glomerulus, for excretion in hemichordates.