Horta

"We are dealing with a silicon creature of the deep rocks. Capable of moving through solid rock as easily as we move through the air."

- Mr. Spock about the Horta in "The Devil in the Dark".

Hortas are sapient, silicon-based lifeforms native to the caves of planet Janus VI. They secrete a powerful, corrosive acid, which allows them to dig perfectly round tunnels and move through them while feeding on rocks and minerals. They do not breathe oxygen, but are not harmed by it either.

Despite their somewhat sluggish appearance, the Hortas have demonstrated great speed and flexibility, as well as resistance to phaser shots.

Life cycle
Hortas live for 50,000 years, and then they all die, except for their leader, the Mother Horta, which takes care of the eggs, making sure that the next generation of Hortas will be safe. Horta eggs are brown spheres composed almost entirely out of silicon.

Discovery
Hortas were the first silicon-based lifeforms officially recognized by science. Before their discovery, it was believed that silicon-based life was pure imagination. They were discovered in 2267, when Human miners found the Horta eggs and destroyed them, believing them to be nothing but rock. In order to protect the remaining eggs, the Mother Horta attacked the miners' colony. At the end, it was decided that the Hortas and their eggs would be left alone, and the Hortas agreed to help the miners by creating access to the planet's large mineral deposits.