What internal structure is a functional part of an open circulatory system, which is found in arthropods and mollusks?

Answer: Haemocoels

Explanation:

In the open circulatory system found in arthropods and mollusks, the heart pumps blood into a blood vessel which branches and opens in an internal structure called Haemocoels.

The haemocoel is a highly functional part because it allows

- organs and tissues to be bathed in blood,

- for exchange of materials to occur between the body cells and the blood.

- blood to eventually flows back to the heart.