Final answer:
When the circular muscles contract, the worm becomes shorter and thicker.
Step-by-step explanation:
Muscle contraction is the physiological process by which a muscle generates tension and shortens or contracts, leading to movement or force production. This process is essential for various activities in the human body, including voluntary movements, maintenance of posture, and organ functions. Muscle contractions can be broadly categorized into two types: isotonic and isometric.
When the circular muscles contract, the worm becomes shorter and thicker. Circular muscles, also known as sphincters, are concentrically arranged bundles of muscle fibers that, when contracted, reduce the size of an opening. This contraction causes the worm to shorten and become thicker.