GRcade

This was the second 4th Doctor story and the very first for new producer Philip Hinchcliffe. Like the rest of season 12 these stories were initially commissioned and developed by the previous team. Although credited to Robert Holmes this is in fact a full rewrite of a story supplied by John Lucarotti who had previously written 3 William Hartnell stories. (Marco Polo, The Aztecs and The Massacre of St Bartholomew's Eve)
In episode 3 (about 7:40) there is a very abrupt cut. This is due to it being considered part of this scene would be too disturbing to children. In would have shown Noah asking Vira to kill him before he turns completely into a Wirrn.
The cliffhanger for part 2 is infamous for its dramatic reveal of...... bubblewrap! But its worth noting the at the time this packaging material was still relatively new and as such possibly not as obvious as it seems today.
Recent stories, Smile and The Beast Below feature colony ships leaving Earth behind. In The Beast Below it leaves just before solar flares hit the Earth. In Smile a character states he is a MedTech.
The Ark in Space is a favourite of both RTD and Stephen Moffat. It's RTD top Doctor Who story and Moffat considers it Tom Bakers best. Its easy spot it influence here to, with Moffat often using the "stolen voice" trick.
A young Ridley Scott worked at the BBC when this story was made. 4 years later he made a film about an alien in space which uses humans as a host. Go figure.
When discussing the 2014 Christmas Special, Last Christmas, was questioned about taking material from the alien films. He replied stating "They never asked Doctor Who to borrow the plot of the Ark in Space". Go figure.

The Wirrn would sadly never be seen again. Although they have returned in a few audio stories to good effect. They are high up on my list of hopes to see given a modern update.

This is cool-