Final answer:
The rhynchocoel is a fluid-filled body cavity that extends from the head to the gut in certain animals and serves as a pocket for the eversible proboscis.
Step-by-step explanation:
The rhynchocoel is a fluid-filled body cavity that extends from the head to nearly two-thirds of the length of the gut in certain animals. It is a unique characteristic of this phylum and serves as a pocket for the eversible proboscis, which is used for food capture. The rhynchocoel can be extended and retracted by specific muscles.