A siphonal notch is a feature of the shell

Gastropod shell

The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

 anatomy in some sea snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 gastropod mollusks.

In these particular groups of sea snails the animal has a soft tubular anterior extension of the mantle

Mantle (mollusc)

The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

 called a siphon through which water is drawn into the mantle cavity and over the gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 and which serves as a chemoreceptor to locate food. This siphonal opening also serves for the exit of the water that has entered by the branchial opening.

The siphonal notch is a noticeable notch situated at the most posterior part of the aperture

Aperture (mollusc)

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

 of the shell, through which the siphon is extended when the animal is active. The notch at the posterior end of the aperture is also called the anal notch, anal sinus, anal canal or posterior canal.

Instead of a simple siphonal notch, some gastropods have an elongated siphonal canal

Siphonal canal

Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water is drawn into the mantle cavity and over the gill and which serves as a chemoreceptor to locate food. In many carnivorous snails, where the siphon is particularly long, the structure...

, a hard shell tube which extends out from the anterior edge of the aperture.