Lt. Radha is both the first explicitly Hindu character (shown by the bright red dot on her forehead, known as a Bindi), and the first Enterprise helmswoman, to appear in Star Trek.
The center section of D'Amato's tricorder differs substantially from the standard Starfleet model. Instead of tape discs and a moiré pattern, it features an intermittently glowing white panel and what appears to be a tubular sensor. In deference to D'Amato's specialty, some prop-conscious fans have dubbed this a "geological tricorder."
Second and final appearance of Dr. M'Benga.
A new access tube was created for this episode to show where the matter-antimatter reaction chamber was.
Sulu discusses the silicon-based creatures on Janus VI, i.e., the Horta of The Devil in the Dark (1967).