Final answer:
Acorn worms have pharyngeal slits, also known as gill slits, for gas exchange.
Step-by-step explanation:
Acorn worms, also known as hemichordates, have pharyngeal slits as their respiratory structure. These slits are also referred to as gill slits. The pharyngeal slits are found in the pharyngeal region of the worm and are responsible for gas exchange. They allow water to enter the body cavity and exchange oxygen and carbon dioxide with the surrounding environment.