A fasciole in

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

s refers to a band or ribbon-like structure that is often seen as a flattened, slightly depressed area on the

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

, formed by the successive growth lines on the edges of a canal. It is typically found on the surface of the shell near the

siphonal canal The siphonal canal is an anatomical feature of the shells of certain groups of sea snails within the clade Neogastropoda. Some sea marine gastropods have a soft tubular anterior extension of the mantle called a siphon through which water i ...

or around 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 ...

. The fasciole can play a role in the shell's overall structure and appearance. In some species, it might help in identifying the shell or understanding its functional morphology. The term "anal fasciole" was proposed by

W.H. Dall William Healey Dall (August 21, 1845 – March 27, 1927) was an American naturalist, a prominent malacologist, and one of the earliest scientific explorers of interior Alaska. He described many mollusks of the Pacific Northwest of North America, ...

in 1885 to describe the raised ring or band that marks the track of the notch, a prominent feature in the sculpture of some

gastropod Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

shells. This structure is particularly notable in turban snails (family

Turbinidae Turbinidae, the turban snails, are a family of small to large marine gastropod molluscs in the superfamily Trochoidea.Bouchet, P. (2014). Turbinidae Rafinesque, 1815. Accessed through: World Register of Marine Species at http://www.marinespecie ...

). The notch serves to allow waste matter to escape from the anus without contaminating the water flowing to the gills. The anal fasciole is often well-defined and can vary in size and prominence among different species. The term "siphonal fasciole", named by

Theodore Gill Theodore Nicholas Gill (March 21, 1837 – September 25, 1914) was an American ichthyologist, mammalogist, malacologist, and librarian. Career Born and educated in New York City under private tutors, Gill early showed interest in natural hist ...

, bears a somewhat similar relation to the anterior end of the

, but is generally less distinctly marked off from the rest of the surface. The notch itself is a break or irregularity in the peristome, indicating the position of the siphon. The term "anal notch" is used to distinguish it from the "siphonal notch".Dall, W. H. (1885). New or specially interesting shells of the Point Barrow Expedition. Proceedings of the United States National Museum. 7(460): 523-526.

References

{{reflist Gastropod anatomy