There is an outtake on the Star Trek Blooper Reel showing the alien parasite attaching itself to Spock's buttocks by mistake.
William Shatner portrays Kirk's brother Sam in the scene where McCoy rolls his body over to identify him. The shot is brief, but freezing the frame reveals Shatner in light character makeup and a mustache.
The Deneva outdoor scenes were shot at the headquarters of TRW Space and Defense Park in Redondo Beach, California (currently [2021] the Northrop Grumman Space Technology headquarters). The establishing shot of Kirk's brother - Sam's lab was a building on the campus of UCLA, and the entrance of the building was the cafeteria at TRW. See Google Earth for location. The actual location where Spock is attacked by the parasite is the lobby of TRW Building E1 next door to the cafeteria where the outdoor scenes were shot. It is now (2021) Northrop Grumman Aerospace Building E1.
This is the first time McCoy's lab is seen. Inside the prop used previously as Balok's lamp device in The Corbomite Maneuver (1966) can be seen sitting on a shelf. Different components of sickbay were added over the first season, such as the decompression chamber seen in Space Seed (1967). Also, one of the life support canisters used on the Botany Bay is present.
Wah Chang designed and constructed the parasite creatures. In the Star Fleet Medical Reference Manual, it was called a "blastoneuron."