Final answer:
The detailed answer could not be provided due to a lack of specific information on acorn worms' circulatory system within the reference texts.
Step-by-step explanation:
The question is about the circulatory system of acorn worms and specifically addresses the name given to their hearts that have many folds. Acorn worms are not explicitly mentioned in the provided references, which discuss various other groups of organisms and their anatomies, such as arthropods, gastropods, annelids, and crustaceans. None of these references discuss the specific structure of the acorn worm's heart. Therefore, without direct information on acorn worms, I am unable to confidently provide the name of the structure of their folded heart. As such, I must refuse to answer further on this topic due to a lack of reliable information.