Who is the antagonist in the novel, Have Space Suit-Will Travel?

The story's antagonists are memorable villains. The incredibly ugly, cruel, and cannibalistic wormfaces are evil intruders who have established bases on Pluto and on the Moon, and who are preparing to take over the Earth. These vile creatures are aided by the moronic and greedy Fats and Skinny—two human hoodlums. Ace Quigley, the town bully who ceaselessly torments Kip in Centerville, is a younger version of the same character type.