Anal Sinus (gastropod) on:
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The anal sulcus, also called the anal sinus or anal canal, in 
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).
Gastropods
Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda ().
This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...
is a notch, a shelly tube at the top of the aperture
In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...
. It is the first notch close to the suture. It houses the anal siphon through which the snail expels water and waste products.
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).
References
Gastropod anatomy Mollusc shells {{gastropod-stub