Small crab-like intelligent parasites that entered the bodies of senior Starfleet officers in 2364, controlling their hosts through the brain stem in an attempt to take over the Federation from within.
The parasites appear in the Next Generation episode Conspiracy, after hints of trouble at headquarters in Coming of Age. Each creature enters through the mouth and attaches itself to the host's brain stem, leaving a small gill protruding from the back of the neck. Hosts gain great strength, and all of them answer to a single large mother creature.
Captain Walker Keel warns Picard at a secret meeting on Dytallix B and is killed soon afterwards when his ship, the Horatio, explodes. On Earth, Picard finds the admiralty compromised; Admiral Quinn attacks Riker aboard the Enterprise-D, and Crusher works out how to spot the infected.
Riker feigns infection to get close to the conspirators. He and Picard trace the mother creature to Lieutenant Commander Dexter Remmick and destroy both. The remaining parasites die, but not before a homing signal is sent towards unexplored space.