Siphonal notch

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A siphonal notch is a small notch at the back edge of the shell opening in some sea snails. The snail has a siphon, a tubular extension of the mantle, which it uses to pull in water to sense chemicals and locate food, and to breathe as water exits through the same opening. The notch allows the siphon to extend out when the animal is active. This rear notch is also called the anal notch, anal sinus, anal canal, or posterior canal. Some snails have a longer siphonal canal—an elongated shell tube that extends from the opening.