Final answer:
Arrow worms, or chaetognaths, have adaptations such as spines or hooks, sensory bristles, and longitudinal muscles that help prevent their heads from flopping around.
Step-by-step explanation:
Arrow worms, also known as chaetognaths, have several adaptations that help their heads from flopping around. One of these adaptations is the presence of spines or hooks on their heads. These structures provide stability and prevent excessive movement. Another adaptation is the presence of sensory bristles or tentacles, which help the arrow worms to sense their environment and stabilize their heads. Additionally, arrow worms have longitudinal muscles that allow them to control the movement of their bodies and heads, preventing flopping.