What was I saying? Oh! Devil in the Dark. Great episode. I'm shocked how well it holds up, even knowing how everything turns out, there's still suspense and intrigue in spades. Much credit has to go to Shatner and Nimoy especially. Nimoy completely sells his mind-meld with the Horta. That scene could have been so awful.
The Horta has grown a subcult in trek fandom; Hortas have appeared as Federation crew members in several novels, and there's also the Trek Punk band: No Kill I
While the Horta does look cheap, it could have been so. Much. Worse.
Classic TWOP recap is here.