..they are plastic with end caps that unscrew.
I occasionally use these myself. I always had misgivings about how secure the caps were.. though I never really had a problem. To satisfy my paranoia, after screwing the cap on, I take a small patch of blue painters tape and put on that end of the cassette so the tape barely overlaps the edge of the cap. With a marker or pen, you can write something if you wish. I just put the number of frames loaded since I vary that somewhat depending on what I'm doing.