Final answer:
A robin struggles to pull an earthworm out because of the worm's chitinous bristles called setae or chaetae, which help in the worm's movement and soil anchorage.
Step-by-step explanation:
A robin may have difficulty pulling an earthworm from its burrow due to the earthworm's small chitinous bristles known as setae or chaetae.
These bristles are anchored in the epidermis and assist the earthworm with locomotion and anchorage in the soil. In the class Oligochaeta, to which earthworms belong, these structures are less numerous than in the closely related class Polychaeta, but still serve an important function in the earthworm's interaction with its environment.