Dear Sonja:

     I'm not sure whether this is an "urban legend" and I don't think
     it applies to the shells you are using but there are some cone
     shells that use a poison harpoon to sting and catch fish for food.
     The poison of some species are toxic enough to kill humans.  I've
     heard "stories" of scuba divers who've surfaced in great pain or
     with minor paralysis.  At first people wouldn't know what happened
     until they later found a living cone shell stuffed into the divers
     wet suit.

     Ted