The Mother Horta

Captain Kirk discovers a chamber filled with thousands of the silicon nodules and the creature causes the roof to collapse, trapping him. Kirk contacts Spock, who, in a reversal of their previous positions, urges Kirk to kill the creature immediately. However, Kirk realises that the creature is not threatening him and begins to "talk" to it. The creature turns, showing him a large wound in its side.
Spock finds a way into the cavern and attempts a Vulcan mind meld but cannot complete it as the creature is in agony from its wound. However, he does learn that it calls itself a Horta. The Horta also gains enough knowledge from the experience to be able to etch the words "NO KILL I" into a nearby rock. However, Kirk and Spock aren't sure if that means, "Please don't kill me", or "I won't kill you".
Kirk orders Dr. McCoy to come and help the creature while Spock mind melds with it again. Spock discovers that every 50,000 years the entire race of Horta dies, except for one creature who remains to protect the eggs and act as their mother.