The gliding slugworm is a split from the slugworm. The pressure of having the dangerous mantiskipper hunt it made the slugworm travel to the beaches, where it came across a new food source: stickyglobes.
This plant, having turned the beaches into things resembling a forest, was in ample supply: the slugworm did not let this opportunity stay put. It then changed its main diet from rotting material to fresh stickyglobes. This excess in hydrogen enabled excessive buoyancy in the slugworms. They used this to their advantage, gaining a membrane in between their "smelling tentacles", which allowed them to climb to the top of a stickyglobe, eat it, then glide to the next one. This behavior allowed them to survive, even if the stickyglobe they were living on floated away, they just had to jump off, land on the next one, and keep eating.