Final answer:
Arrow worms, or chaetognaths, have a tough cuticle on their body surface that prevents expansion. The fluid-filled pseudocoelom in arrow worms serves as a hydrostatic skeleton, providing stiffness to their body. This allows them to move efficiently.
Step-by-step explanation:
Arrow worms, also known as chaetognaths, have a tough covering called cuticle on their body surface. This cuticle prevents their body from expanding, allowing the buildup of fluid pressure in a body cavity called the pseudocoelom. This fluid-filled cavity serves as a hydrostatic skeleton, providing stiffness to the arrow worm's body. The hydrostatic skeleton allows the worm to move efficiently along solid surfaces.