Final answer:
Harry E. Keller's reason for favoring arms over tentacles in aliens is based on the adaptations that support efficient land movement, where arms enable a pendulum-like stride, indicating that the structure of arms is well-suited for terrestrial locomotion.
Step-by-step explanation:
According to Harry E. Keller, a supporting reason for aliens to have arms over other appendages like tentacles is primarily related to the efficiency of movement on land. The evolution of the appendicular skeleton of early tetrapods shows a transition from limbs splayed out to the side, which required a large expenditure of energy for movement, to limbs placed under the body facilitating a more efficient pendulum-like stride. This structural adaptation in limbs, with decreased adductor muscle size and an increased range of motion of the scapulae, indicates that having arms with an orientation that supports forward motion is advantageous for terrestrial locomotion.
The structural advantages of arms are evident when comparing them to other anatomical forms, such as the feeding tentacles of aquatic animals, which serve different functions such as sensing and feeding but are less adapted to efficient movement on land. This is further supported by the discussion on paired appendages, showing the significant role of gravity, leverage, and locomotive efficiency in shaping the anatomy of land-dwelling creatures.