A plisagraph is a sensor instrument, similar to a tricorder, designed specifically for the study and examination of Trill symbionts and similar lifeforms. (DS9 - The Lives of Dax short story: "Sins of the Mother") In the year 2376, Ezri Dax used a plisagraph to scan for Senator Rylen Talris after he'd been taken hostage in the Trill Senate Tower in Leran Manev. (DS9 - Worlds of DS9 novel: Trill: Unjoined)