Final answer:
The internal coelom of acorn worms is filled with a fluid called the pseudocoelom, which serves as a hydrostatic skeleton. It is divided into segments along the length of the worm.
Step-by-step explanation:
The internal coelom of acorn worms is filled with a fluid. This fluid-filled body cavity serves as a hydrostatic skeleton, providing internal support and allowing movement of the worm's muscles. It is called a pseudocoelom. The pseudocoelom is divided into segments along the length of the worm.