Final answer:
The proboscis in acorn worms is supported by a rhynchocoel, a fluid-filled cavity. This cavity aids in the extension and retraction of the proboscis, which is used for food capture.
Step-by-step explanation:
The proboscis in acorn worms is supported by a fluid-filled cavity called rhynchocoel extends from the head to two-thirds the length of the gut and aids in the extension and retraction of the proboscis. The proboscis itself is a harpoon-like structure used for food capture, which can be extended by hydrostatic pressure and retracted by a retractor muscle.