Cecilioides acicula is a tiny land snail. It is native to Europe, but has been introduced to other parts of the world, including the U.S. (see this post for a record from Virginia).
I was skimming thru an old paper* about Cecilioides acicula earlier today when I noticed the following photograph of what appears to be a backlit shell.
What attracted my attention was the zig-zag path of the columella, the central axis of the shell. In the righthand copy I highlighted the columella with a yellow broken line.
I was under the impression that in most species the columella was straight. Here is a photo showing the columella of Albinaria anatolica (red arrows).
I may be totally mistaken, of course, and zig-zag columellas similar to that of Cecilioides may be more common.
*Wächtler, W. 1929. Anatomie und Biologie der augenlosen Landlungenschnecke Caecilioides acicula Müll. Z. f. Morphol. u. Ökol. d. Tiere 13:24.