Young Brunette Girl Wearing A Seashell Necklace

Some polychaetes, marine annelid worms in the family Serpulidae, secrete a hard tube made of calcium carbonate, adhering to stones or other shells. This tube resembles, and can be confused with, the shell of marine gastropod mollusks in the family Vermetidae, the worm snails.
Other more atypical kinds
A few other categories of marine animals leave remains which might be considered "seashells" in the widest possible sense of the word.