The Cragie tube or Craigie tube is a method used in microbiology for determining bacterial motility.
Cragie tube Wikipedia
A hollow tube with some culture medium is placed in semi-solid agar inside a bottle. Now a sample of the bacterium to be tested is inoculated into the medium in the hollow tube and the setup is incubated at 37°C overnight.
On examining the areas where bacterial growth has occurred we find that:-the colonies of the non-motile bacteria remain confined within the tube at the site of inoculation
the motile bacteria swim out from the bottom of the tube and colonize the surrounding medium as well
Confirmation may be obtained by subculture and retesting.