Final answer:
The endostyle is considered a forerunner of the vertebrate thyroid gland because it produces substances similar to thyroid hormones and serves similar functions. It undergoes modifications in vertebrates to become the thyroid gland.
Step-by-step explanation:
The endostyle, a strip of ciliated mucus-producing tissue in the pharynx, is considered a forerunner of the vertebrate thyroid gland because it produces substances similar to thyroid hormones. In vertebrates, the endostyle is homologous with the thyroid gland and serves similar functions. The endostyle in invertebrate chordates is used to filter food particles, while in vertebrates it undergoes modifications to become the thyroid gland, which produces thyroid hormones that regulate metabolism.