They are failed experiments of Professor Monkey-For-A-Head, and the Professor unleashes the mutants upon Earthworm Jim in repeated efforts to reclaim the Super Suit which the Professor designed, and which Jim is wearing.
The mutant experiments are airborne, floating green blobs, whose only discernible feature are their single eye-stalk. They can cause damage to Jim's Super Suit upon contact, and will make a beeline for Jim immediately upon seeing him. Some solitary mutant experiments are encountered, but most of them hunt Jim in packs.
Although easily dispatched with Jim's Plasma Blaster the mutant experiments are filled with mutant worms, which will explode out of it over the floor when it is blasted. These little purple worms can still cause damage and will continue to quickly wriggle towards Jim, and must be dispatched also, causing numerous hostiles from one enemy.