Editing Gastropoda (section)

== Anatomy ==
[[File:Snail diagram-en edit1.svg|thumb|The anatomy of a common air-breathing land snail: much of this anatomy does not apply to gastropods in other [[clade]]s or groups.]]

Snails are distinguished by an anatomical process known as [[Torsion (gastropod)|torsion]], where the visceral mass of the animal rotates 180° to one side during development, such that the [[anus]] is situated more or less above the head. This process is unrelated to the coiling of the shell, which is a separate phenomenon. Torsion is present in all gastropods, but the [[Opisthobranchia|opisthobranch gastropods]] are secondarily untorted to various degrees.<ref name="Southern Synthesis">{{cite book|last=Kay|first=A.|title=Mollusca: The Southern Synthesis. Fauna of Australia|year=1998|publisher=CSIRO Publishing|isbn=978-0-643-05756-2|pages=565–604|author2=Wells, F. E. |author3=Poder, W. F. |editor=Beesley, P. L. |editor2=Ross, G. J. B. |editor3=Wells, A.|chapter=Class Gastropoda}}</ref><ref name="Brusca&Brusca">{{cite book|last=Brusca|first=R. C.|title=Invertebrates|year=2003|publisher=Sinauer Associates, Inc.|isbn=978-0-87893-097-5|pages=701–769|author2=Brusca, G. J.|chapter=Phylum Mollusca}}</ref>

Torsion occurs in two stages. The first, mechanistic stage is muscular, and the second is [[:wiktionary:mutagenetic|mutagenetic]]. The effects of torsion are primarily physiological. The organism develops by asymmetrical growth, with the majority of growth occurring on the left side. This leads to the loss of right-side anatomy that in most bilaterians is a duplicate of the left side anatomy. The essential feature of this asymmetry is that the anus generally lies to one side of the median plane. The [[Ctenidium (mollusc)|gill-combs]], the [[osphradium|olfactory organs]], the [[hypobranchial gland|foot slime-gland]], [[Nephridium|nephridia]], and the [[Atrium (heart)|auricle]] of the heart are single or at least are more developed on one side of the body than the other. Furthermore, there is only one [[genital]] orifice, which lies on the same side of the body as the anus.<ref>{{Cite book|url=https://www.biodiversitylibrary.org/item/16786|title=Manual of the New Zealand mollusca /|first=Henry|last=Suter|publisher=J. Mackay, govt. printer|access-date=2018-01-14|archive-url=https://web.archive.org/web/20170926235652/http://www.biodiversitylibrary.org/item/16786|archive-date=2017-09-26|url-status=live}}</ref> Furthermore, the anus becomes redirected to the same space as the head. This is speculated to have some evolutionary function, as prior to torsion, when retracting into the shell, first the posterior end would get pulled in, and then the anterior. Now, the front can be retracted more easily, perhaps suggesting a defensive purpose.{{citation needed|date=February 2025}}

Gastropods typically have a well-defined [[head (anatomy)|head]] with two or four sensory [[tentacle]]s with eyes, and a ventral foot. The foremost division of the foot is called the propodium. Its function is to push away sediment as the snail crawls. The larval shell of a gastropod is called a [[protoconch]].{{citation needed|date=February 2025}}

=== Shell ===
[[File:Zonitoides nitidus shell (cropped).jpg|thumb|The shell of ''[[Zonitoides nitidus]]'', a small land snail, has [[Gastropod shell#Chirality|dextral coiling]], which is typical (but not universal) of [[gastropod shell]]s.]]
[[File:Physella_acuta_01.JPG|thumb|The shell of ''[[Physella acuta]]'', a freshwater snail, which is left-coiling ([[Gastropod shell#Chirality|sinistral]]).]]
{{main|Gastropod shell}}

Most shelled gastropods have a one piece [[Gastropod shell|shell]] (with exceptional [[Juliidae|bivalved gastropods]]), typically coiled or spiraled, at least in the larval stage. This coiled shell usually opens on the right-hand side (as viewed with the shell [[apex (mollusc)|apex]] pointing upward). Numerous species have an [[operculum (gastropod)|operculum]], which in many species acts as a trapdoor to close the shell. This is usually made of a horn-like material, but in some molluscs it is calcareous. In the land slugs, the shell is reduced or absent, and the body is streamlined.

Some gastropods have adult shells which are bottom heavy due to the presence of a thick, often broad, convex ventral callus deposit on the inner lip and adapical to the aperture which may be important for gravitational stability.<ref>{{cite journal|author1=Geerat J. Vermeij|title=The balanced life: evolution of ventral shell weighting in gastropods|journal=Zoological Journal of the Linnean Society|volume=194|issue=1|year=2022|pages=256–275|url=https://doi.org/10.1093/zoolinnean/zlab019|doi=10.1093/zoolinnean/zlab019|access-date=2022-01-04|archive-date=2023-08-23|archive-url=https://web.archive.org/web/20230823114009/https://academic.oup.com/zoolinnean/article/194/1/256/6261015|url-status=live}}</ref>

=== Body wall ===
Some [[Opisthobranchia|sea slugs]] are very brightly colored. This serves either as a [[Aposematism|warning]], when they are [[poisonous]] or contain [[stinging cell]]s, or to [[camouflage]] them on the brightly colored [[hydroids]], [[sponge]]s, and seaweeds on which many of the species are found.{{citation needed|date=February 2025}}

Lateral outgrowths on the body of [[nudibranch]]s are called [[cerata]]. These contain an outpocketing of [[digestive gland]]s called the [[Diverticula (mollusc)|diverticula]].{{citation needed|date=February 2025}}

=== Sensory organs and nervous system ===
[[File:Vinogradski puz glava.jpg|thumb|The upper pair of [[tentacle]]s on the head of ''Helix pomatia'' have eye spots, but the main sensory organs of the snail are sensory receptors for [[olfaction]], situated in the [[epithelium]] of the tentacles.]]
{{main|Sensory organs of gastropods|Nervous system of gastropods}}

The [[sense|sensory organs]] of gastropods include [[olfaction|olfactory organs]], eyes, [[statocyst]]s and [[mechanoreceptor]]s.<ref name="sensory"/> Gastropods have no hearing.<ref name="sensory">Chase R.: ''Sensory Organs and the Nervous System''. in Barker G. M. (ed.): ''The biology of terrestrial molluscs''. CABI Publishing, Oxon, UK, 2001, {{ISBN|0-85199-318-4}}. 1-146, cited pages: 179–211.</ref>

In terrestrial gastropods (land snails and slugs), the olfactory organs, located on the tips of the four [[tentacle]]s, are the most important sensory organ.<ref name="sensory"/> The chemosensory organs of [[opisthobranch]] marine gastropods are called [[rhinophore]]s.

The majority of gastropods have simple visual organs, eye spots either at the tip or [[Leptopoma|base]] of the tentacles. However, "eyes" in gastropods range from simple [[ocelli]] that only distinguish light and dark, to more complex [[Eye#Pit eyes|pit eyes]], and even to [[Eye#Spherical lens eye|lens eyes]].<ref>{{cite book | last=Götting | first=Klaus-Jürgen | title= Lexikon der Biologie | chapter=Schnecken | editor=Becker, U. |editor2=Ganter, S. |editor3=Just, C. |editor4=Sauermost, R. | year=1994 | publisher=Spektrum Akademischer Verlag | location=Heidelberg | isbn=978-3-86025-156-0}}</ref> In land snails and slugs, vision is not the most important sense, because they are mainly [[Nocturnality|nocturnal]] animals.<ref name="sensory"/>

The nervous system of gastropods includes the [[peripheral nervous system]] and the [[central nervous system]]. The central nervous system consists of [[ganglia]] connected by nerve cells. It includes paired ganglia: the cerebral ganglia, pedal ganglia, [[Osphradium|osphradial]] ganglia, pleural ganglia, parietal ganglia and the visceral ganglia. There are sometimes also buccal ganglia.<ref name="sensory"/>

=== Digestive system ===
{{main|Digestive system of gastropods|Radula}}
The [[radula]] of a gastropod is usually adapted to the food that a species eats. The simplest gastropods are the [[limpet]]s and [[abalone]], herbivores that use their hard [[radula]] to rasp at [[seaweed]]s on rocks.{{citation needed|date=February 2025}}

Many marine gastropods are burrowers, and have a [[siphon (mollusc)|siphon]] that extends out from the [[Mantle (mollusc)|mantle]] edge. Sometimes the shell has a [[siphonal canal]] to accommodate this structure. A siphon enables the animal to draw water into their [[mantle cavity]] and over the gill. They use the siphon primarily to "taste" the water to detect prey from a distance. Gastropods with siphons tend to be either predators or scavengers.{{citation needed|date=February 2025}}

=== Respiratory system ===
{{main|Respiratory system of gastropods|Respiratory system#Anatomy of respiratory system in invertebrates}}
Almost all marine gastropods breathe with a [[gill]], but many freshwater species, and the majority of terrestrial species, have a pallial [[lung]]. The respiratory protein in almost all gastropods is [[hemocyanin]], but one freshwater [[Pulmonata|pulmonate]] family, the [[Planorbidae]], have [[hemoglobin]] as the respiratory protein.{{citation needed|date=February 2025}}

In one large group of sea slugs, the gills are arranged as a rosette of feathery plumes on their backs, which gives rise to their other name, [[nudibranch]]s. Some nudibranchs have smooth or [[wart]]y backs with no visible gill mechanism, such that respiration may likely take place directly through the skin.

=== Circulatory system ===
{{main|Circulatory system of gastropods}}
Gastropods have [[Circulatory system#Open circulatory system|open circulatory system]] and the transport fluid is [[hemolymph]]. [[Hemocyanin]] is present in the [[hemolymph]] as the respiratory pigment.{{citation needed|date=February 2025}}

===Excretory system===
{{main|Excretory system of gastropods}}
The primary organs of excretion in gastropods are [[nephridium|nephridia]], which produce either [[ammonia]] or [[uric acid]] as a waste product. The nephridium also plays an important role in maintaining water balance in freshwater and terrestrial species. Additional organs of excretion, at least in some species, include pericardial glands in the body cavity, and digestive glands opening into the [[stomach]].{{citation needed|date=February 2025}}

=== Reproductive system ===
[[File:Elysia timida mating.jpg|thumb|Mating behaviour of ''[[Elysia timida]]'']]
{{main|Reproductive system of gastropods}}
[[Mating of gastropods#Courtship|Courtship]] is a part of [[mating of gastropods|mating]] behavior in some gastropods, including some of the [[Helicidae]]. Again, in some land snails, an unusual feature of the [[reproductive system of gastropods]] is the presence and utilization of [[love dart]]s.{{citation needed|date=February 2025}}

In many marine gastropods other than the [[opisthobranch]]s, there are separate sexes ([[dioecious]]/[[Gonochorism|gonochoric]]); most land gastropods, however, are [[hermaphrodite]]s.{{citation needed|date=February 2025}}