[4] They live in groundwater in karstic areas of southern France and western Switzerland.

[5] Gelyella shows some paedomorphosis, in which animals reach sexual maturity while still partly resembling juveniles.

[4] The adults are 300–400 micrometres (0.012–0.016 in) long with a nearly cylindrical body that tapers towards the rear.

[5] A second species was later collected from the Gorges de l'Areuse in the Swiss Jura,[5] and named Gelyella monardi to mark the centenary of the birth of the Swiss naturalist Albert Monard.

[6] It is thought that the two species may have begun to diverge from a marine common ancestor following the changes in coastline during the Miocene.