The first gorilla appearance.
The title is based on the song "Diamonds Are a Girl's Best Friend" from the musical "Gentlemen Prefer Blondes" written by Jule Styne and Leo Robin, which became the theme song of Carol Channing.
This marks the first of many uncredited appearances by the late Janos Prohaska (the gorilla) who went on to play several other apes and a giant spider. He was also the Horta and the Mugato and a Tiger in Star Trek: The Original Series (1966)
First time a gorilla is seen on the island. Second is #2.2;Beauty Is As Beauty Does. Third is #2.11;Chain of Command. Fourth is #2.22; Forward March. Fifth is #3.20; Our Vines Have Tender Apes. The one on "Beauty is as Beauty Does" was female; Gilligan naming her "Gladys", so there was more than one on the island.
In addition to Carol Channing as mentioned, "Diamonds are a Girl's Best Friend" (along with the Broadway/movie source's title tune "Gentlemen Prefer Blondes"), became the theme song(s) for Marilyn Monroe.