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univalve shell 
Land or aquatic mollusk having a foot and head, which retract into a spiral shell made of a single piece.
morphology of a univalve shell 
spiral rib 
Protruding line on the whorl of the shell parallel to the suture line.
whorl 
Each of the turns around the apex, except the last; its diameter grows.
siphonal canal 
Tubular extension of the opening of the last complete turn allowing water to circulate.
inner lip 
Inner wall of the opening made by the last complete turn of the shell, opposite the outer lip.
columella fold 
Small protuberance located along the columella.
axial rib 
Protruding line on the whorl of the shell parallel to the columella.
outer lip 
Outer wall of the opening made by the last complete turn of the shell, opposite the inner lip.
aperture 
Orifice of the last complete turn allowing the head and foot of the mollusk to exit; it also withdraws into its shell through this opening.
suture 
Line marking off the successive whorls of the shell.
columella 
Solid axis of the shell that can be solid or hollow; the whorls turn around it.
nuclear whorl 
First complete turn of the shell formed at the larval stage.
apex 
Crown from which the shell grows.