Anal sulcus

The anal sulcus, also called the anal sinus or anal canal, in Gastropods is a notch, a shelly tube at the top of the aperture.[1] It is the first notch close to the suture. It houses the anal siphon through which the snail expels water and waste products.

Shell of Drillia poecila Sysoev & Bouchet, 2001, showing the anal sulcus on top of the aperture

The anal sulcus can be described, according to the species, as well-defined, weakly defined, shallow, sharp, wide, narrow or inverted U-shaped. In some species, such as in the Murex family, the anal sulcus is absent.

The anal sulcus can be linked on the edges to a fasciole, a spiral band on the shell, formed by successive growth lines. It can also have a subsutural callus (such as in Clathrodrillia callianira).