Question

structures that produce and release substances into a duct are known as \boxed{} glands.

Explanation:

In biology (a subfield of Natural Science), exocrine glands are structures that produce and release substances into a duct. The question is about identifying the type of gland with this characteristic, and the correct term is exocrine (to fill in the blank, the full answer for the structure is exocrine glands, but the blank here is for the type of gland, so the answer is exocrine).

Answer:

exocrine