... host. In exchange for food, oxygen and comfort, the brain-symbiote must generate hormones and enzymes that enable the magter to survive. Some of these might aid digestion, enabling the magter to eat any plant or animal life they can lay their hands on. The symbiote might produce sugars, scavenge the blood of toxins—there are so many things it could do. Things it must have done, since the magter are obviously the dominant life form on this planet. They paid a high price for the symbiote, but it didn't matter to race survival until now. Did you notice that the magter's brain ... — Planet of the Damned • Harry Harrison
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