Some of the shots of Earth shown to Spock on a computer screen as he is melding with the cloud intelligence, of children running with a dog, were taken from stock footage of Lassie's Rescue Rangers (1973), a Filmation TV show simultaneous in production.
Lieutenant Arex appears for the second time here, and it is the first where he speaks.
This is where we see, for the first time, the Enterprise's antimatter nacelles, where Kirk and Scotty use the piece of villi extracted from the cloud-creature to recharge the antimatter engines.
Though Robert Wesley was played by John Duke in The Ultimate Computer (1968), James Doohan provided his voice for the cartoon. Also, instead of the gold command tunic he wore then, here he can be seen wearing a blue one.
The cloud creature is an amalgamation of The Doomsday Machine (a huge thing wandering into our galaxy that digests planets), The Immunity Syndrome (a huge living thing wandering into our galaxy that drains power from the Enterprise and which the Enterprise enters), and Obsession (a deadly living cloud capable of space travel).