This page contains information regarding Star Trek: Discovery, and thus may contain spoilers.
The trance worm was a large, predatory semi-aquatic animal. They had the ability to mesmerize their prey by locking eyes with them, which was evidenced by a blue glow. They lived in groups and had a breeding cycle.
Trance worms were once widespread, but their numbers dwindled drastically after the United Federation of Planets collapsed in the 31st century and could no longer enforce the Endangered Species Act. Fresh trance worm was considered a delicacy.
In 3188, Cleveland Booker stole a trance worm he named "Molly" from Cosmo Traitt on Hima. After a confrontation during which Molly devoured Ithor and Ithyk, Molly briefly swallowed Michael Burnham before Booker convinced her to regurgitate her. Book then delivered her to Sanctuary Four, where a number of other trance worms were sheltered. (DIS: "That Hope Is You, Part 1")