Star Trek: The Original Series, Season 1

Spock:
[presenting the first findings on the alien creature] Interesting, gentlemen. A one-cell creature resembling, more than anything else, a huge, individual brain cell.

Kirk:
Yes. That would answer a lot of questions.

Spock:
Do you understand what I'm suggesting, Captain?

Kirk:
I think so. This may be one cell in a larger organism, an incredibly huge organism, in fact.

Spock:
And although it is not physically connected to the other cells, it is, nevertheless, part of the whole creature, guided by the whole, drawing its strength from the whole, which probably accounts for its unusual resistance to our phaser weapons.