My oldest sister adopted two hermit crabs when I was about 10 years old. They had pretty painted shells and lived in a small tank with a slatted top. If I remember correctly, they didn’t even have any sand in their cage, just tiny little rocks.
I got two hermit crabs of my own when I was a freshman in college. I figured they’d be a good dorm-room pet since they were so low maintenance, as I had learned from my sister. You put a wet sponge in their tank, a little food, a little water, and they’re good to go! I only kept them for a year or two before deciding I didn’t want them anymore and I had my mom take them back to Petco so someone else could adopt them.
Then, about two years ago, I decided I wanted a pet for my office at the school I work at. Not a hamster (smelly escape artists), not fish (you can’t even hold them!). Other mammals were going to be smelly also, and probably take more work than I was willing to give, maybe even needing me to take them home for the weekend. Who knows how that would have turned out with my two cats! I’d never had any reptiles as pets, so I didn’t want to learn that while at work (and those would possibly need to come home with me over long breaks as well).
All that to say: I decided on hermit crabs! I bought 3 or 4 originally, then was given 3 by coworkers who no longer wanted the ones they had at home. Hermit crabs will often bury themselves during the day and also to molt when their exoskeleton gets too small, so I often don’t see all of mine at one time, but I’m pretty sure I have seven in my tank right now.
They are pretty low maintenance, so I would recommend them as an office or classroom pet, but they are a little more work than I knew going into it! Hermit crabs do not belong in hamster cages or those small plastic tanks with the slatted top like my sister had when I was a kid, or what I had when I was in college. They also shouldn’t be housed with only rocks and a small shell full of freshwater, and there shouldn’t be a sponge in there at all. They need lots of heat and humidity (they breathe through gills!), lots of sand to bury themselves under, things to climb on and hide in, and freshwater and saltwater dishes large enough to submerge themselves. I am no expert and I am definitely still learning the best ways to make a great life for these little guys (I actually don’t know their sexes, I’ve never looked), but that’s just some information to get you started! If you are interested in getting hermit crabs of your own (always get more than one, they are social animals), definitely check out the subreddit r/hermitcrabs and their Google Docs and/or the Facebook group Hermit Crab Owners.