Shell-less snails??

[Alexoxoandra]

Excuse the algae mess, but what are those two little ones? I got 2 nerite snails about 2 months ago and one day I see one of those little dudes & one nerite, so I thought maybe his shell fell off (is that even possible?) Or something. I've only ever seen 1 of each until this morning - apparently there's 2 new snails & 2 nerites! My question is- do snails grow shells? Or do they have ones just very small? Also any idea what kind these are? The only thing I have added to the tank has been plants from co-op so could these be some bonus snails? I'm so surprised/confused lol

[Mmiller2001]

Bladder snails. They are hitchhikers. 

[H.K.Luterman]

Those are bladder snails, a very common little snail that often comes in on live plants. They are great at eating algae and extra food, though they are notoriously productive at reproduction.

To answer your question, yes, snails grow their own shells, much like we grow fingernails; they utilize calcium from the environment as well as their food, and often serve as "canaries" on what the mineral levels are in the tank. There is a RARE instance when the muscle that holds a snail inside its shell gets damaged, and then the naked body of the snail becomes detached, and this is always 100% fatal. I've only seen this happen in Mystery snails, and is most likely caused by someone forcefully pulling them off the glass. 

But what you have there are just a different species of snail than the nerite.