Symbion is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-stage life-cycle. They appear so different from other animals that they were assigned their own phylum, Cycliophora, shortly after being discovered in 1995. Cycliophora was the first new phylum of multicelled organism to be discovered since the Loricifera in 1983.

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