Prosobranchia was a large

taxonomic Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

subclass of sea snails, land snails and freshwater snails. This taxon of

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

s dates back to the 1920s. It has however been proven to be polyphyletic (consisting of more than one

lineage Lineage may refer to: Science * Lineage (anthropology), a group that can demonstrate its common descent from an apical ancestor or a direct line of descent from an ancestor * Lineage (evolution), a temporal sequence of individuals, populati ...

of descent). Generally speaking in biology

taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...

is required to reflect

phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...

, in other words the classification of a group must reflect its evolutionary descent, as far as that is known, so the taxon Prosobranchia is no longer considered suitable to be used. One can still encounter this subclass used as if it is still valid in many texts and websites. Although Prosobranchia is no longer generally accepted as a taxon by people who study living Mollusca, still the term prosobranch is legitimately used as an anatomically descriptive adjective or noun, and the taxon Prosobranchia is still sometimes used by paleontologists. ''Prosobranch'' means ''

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s in front'' (of the

heart The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...

). In contrast ''

opisthobranch Opisthobranchs () is now an informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping. Euopisth ...

'' means ''gills behind'' (and to the right of the heart). Prosobranchs have their gills,

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

cavity and

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

situated in front of their heart. Most prosobranchs have separate sexes. The majority of

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

gastropods are prosobranch, as are a few land snails and freshwater snails. The prosobranch gastropods include the majority of marine snails, among them

conch Conch () is a common name of a number of different medium-to-large-sized sea snails. Conch shells typically have a high spire and a noticeable siphonal canal (in other words, the shell comes to a noticeable point at both ends). In North Am ...

es, cones, cowries,

limpet Limpets are a group of aquatic snails that exhibit a conical shell shape (patelliform) and a strong, muscular foot. Limpets are members of the class Gastropoda, but are polyphyletic, meaning the various groups called "limpets" descended indep ...

s, murexes, periwinkles, volutes and whelks, as well as numerous freshwater groups, and some land snails with an operculum.

Description

The majority of prosobranchs have an operculum, a corneous or

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...

plate situated on the dorsal surface of the foot. In many prosobranchs, the animal can completely close the

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...

with the operculum. The nervous system of prosobranchs is twisted into a figure 8 due to a developmental process known as torsion. The eyes are situated at the base of the

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

Taxonomic context

The taxonomy of the gastropods is changing rapidly. The old classification (

Johannes Thiele Johannes Thiele may refer to: *Johannes Thiele (zoologist) *Johannes Thiele (chemist) {{hndis, Thiele, Johannes ...

) divided Gastropoda into three subclasses: Prosobranchia,

Opisthobranchia Opisthobranchs () is now an informal name for a large and diverse group of specialized complex gastropods which used to be united in the subclass Opisthobranchia. That taxon is no longer considered to represent a monophyletic grouping. Euopisth ...

and

Pulmonata Pulmonata or pulmonates, is an informal group (previously an order, and before that a subclass) of snails and slugs characterized by the ability to breathe air, by virtue of having a pallial lung instead of a gill, or gills. The group includ ...

. The subclass Prosobranchia (

Henri Milne-Edwards Henri Milne-Edwards (23 October 1800 – 29 July 1885) was an eminent French zoologist. Biography Henri Milne-Edwards was the 27th child of William Edwards, an English planter and colonel of the militia in Jamaica and Elisabeth Vaux, a Frenchw ...

) was further divided into the orders

Archaeogastropoda Archaeogastropoda (also known as Aspidobranchia) was a taxonomic order of sea snails used in older classifications of gastropods, i.e. snails and slugs. Archeogastropoda are marine prosobranch gastropod mollusks, mainly herbivores, typically havi ...

Mesogastropoda Mesogastropoda was for many years a traditional taxonomic group of snails, an order. The order was composed mostly of sea snails, but it also included some land snails and freshwater snails, all of which were prosobranch gastropod mollusks. Thi ...

and

Neogastropoda Neogastropoda is an order of sea snails, both freshwater and marine gastropod molluscs. Description The available fossil record of Neogastropoda is relatively complete, and supports a widely accepted evolutionary scenario of an Early Creta ...

. The new version of the gastropod classification is explained in '' Taxonomy of the Gastropoda'' (Ponder & Lindberg, 1997).

References

*Thiele, J., 1929–1935. ''Handbuch der Systematischen Weichtierkunde''. 2 vols. 1154 p., 584 figs. *Bieler, R. & P. M. Mikkelsen (eds.), 1992. ''Handbook of Systematic Malacology, Part 1 (Loricata olyplacophora Gastropoda: Prosobranchia)''.

Smithsonian Institution The Smithsonian Institution ( ), or simply the Smithsonian, is a group of museums and education and research centers, the largest such complex in the world, created by the U.S. government "for the increase and diffusion of knowledge". Founded ...

and

National Science Foundation The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National I ...

, xviii + 625 pp., 470+1 text-fig. (Annotated English-language edition of: Thiele, J., ''Handbuch der systematischen Weichtierkunde'', Teil 1). Also published, in 1993, by Gustav Fischer Verlag, Stuttgart/Jena/New York. * * * {{Cite journal , last1 = Bieler , first1 = R. , year = 1990 , title = Haszprunar's "clado-evolutionary" classification of the Gastropoda - a critique , journal = Malacologia , volume = 31 , issue = 2, pages = 371–380 Obsolete gastropod taxa Gastropod taxonomy Mollusc subclasses