The curve in the centre of the image is the siphonal notch of a Tonna perdix shell. This is the depression in the outer lip of the shell through which an aquatic mollusc can extend its tube-like siphon for the passage of water for a variety of purposes such as locomotion, feeding and respiration as the water passes over the gills.
For Macro Mondays theme 'Curves'. The siphonal notch is just 2cm across as I wanted to get in close so that only the shell was visible. A strong head torch at the posterior end of the shell provided the light that gave strange effects through the translucency of the relatively thin-walled shell.
No snails were harmed in the making of this photograph.