These five anatomically correct lumber vertebrae have twist-off lids concealing small inner compartments.
See here for video: https://youtu.be/X4DSE5UOAG0
The tolerance between the lid and container is intentionally quite tight, so it will take some effort to put the lid on and off the first few times. After the first few times, the material will smooth out and the lid will move much easier but will remain tight enough not to fall off inadvertently.
Note that the lids are specific to the containers, so be sure you have them properly matched!
In order to have smooth surfaces between the lids and containers, the pieces must be printed with the “twist notches” oriented upwards as they are in the files provided. This means that you will need to use support and expect to do some post-print cleaning.
Reference files from BodyParts3D, © The Database Center for Life Science licensed under CC Attribution-Share Alike 2.1 Japan.