ST:TAS #16A - The Eye Of The Beholder
Writer: David P. Harmon
Captain James T. Kirk: William Shatner
Commander Spock: Leonard Nimoy
Dr. Leonard McCoy: De Forest Kelley
Lieutenant Commander Montgomery Scott: James Doohan
Orbiting Laectra VII the Enterprise finds a missing starship, the Ariel. Obviously everybody has left, the entire ship is empty. The ship's records reveal that the whole crew has beamed down to the planet. A landing party, including Kirk and Spock, also beam down to Lactra VII.
There huge slugs are the intelligent life form amongst the other – more sophisticated than humans. They put the landing party in a zoo for exhibition. Only because Spock can reach one of their children by telepathy he is able to convince the slugs that they are intelligent, too. They are given back their freedom.