Probably a silverfish. They feed on cellulose residue from shampoo and shaving cream. They also feed on caulk and glue in your tub and shower walls. They are harmless and can live anywhere in your home. To get rid of them safely, use pyrethrum powder found at hardware stores. It is safe to use around kids and pets.
It's difficult to say without a picture or more information, but I would guess that they are the larvae of some sort of fly, and are probably just feeding off of bacteria or other microbial growth in a wet environment. Most likely harmless, but if they bother you, cleaning with bleach will probably take care of them.