Good question. I'd say it depends on the skill/experience of the magician and the specific nature of the fusion.

Human magicians need spells for magic, although the spells themselves are not really magical by themselves (semi-magical at best), nor do they summon any overworldy powers, they rather serve as mantras to focus the magicians mind and ease the manipulation of mana into the magical effect. So usually a magician would need to be able to speak in order to use magic. The more experienced a magician gets, however, the more he or she is able to use magic without spells, either able to focus enough by just thinking the words, or not needing them at all. However, even powerful magicians can only use low to mid-level magics without spells, the really powerful magics are too complicated to control to do so without the help of spells. That's why half-demons and demons are so powerful, they can feel the mana and thus more easily manipulate it without the need of spells.

The worm's sexdrive would probably make things a bit harder for a magician, too, possibly distracting him/her.