Prosobranchia was a large taxonomic subclass of sea snails, land snails and freshwater snails. This taxon of

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 ...

s dates back to the 1920s. It has however been proven to be

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

(consisting of more than one lineage of descent). Generally speaking in biology

taxonomy image:Hierarchical clustering diagram.png, 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme o ...

is required to reflect

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

, 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 respiration organ, respiratory organ that many aquatic ecosystem, 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 r ...

s in front'' (of the

heart The heart is a muscular Organ (biology), organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make the circulatory system. The pumped blood carries oxygen and nutrie ...

). In contrast '' opisthobranch'' means ''gills behind'' (and to the right of the heart). Prosobranchs have their gills, mantle cavity and

anus In mammals, invertebrates and most fish, the anus (: anuses or ani; from Latin, 'ring' or 'circle') is the external body orifice at the ''exit'' end of the digestive tract (bowel), i.e. the opposite end from the mouth. Its function is to facil ...

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

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 ...

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 (mollusc), spire and a noticeable siphonal canal (in other words, the shell comes to a noticeable point on both ...

es, cones,

cowries Cowrie or cowry () is the common name for a group of small to large sea snails in the family Cypraeidae. Cowrie shells have held cultural, economic, and ornamental significance in various cultures. The cowrie was the shell most widely used wo ...

limpet Limpets are a group of aquatic snails with a conical gastropod shell, shell shape (patelliform) and a strong, muscular foot. This general category of conical shell is known as "patelliform" (dish-shaped). Existing within the class Gastropoda, ...

murex ''Murex'' is a genus of medium to large sized predatory tropical sea snails. These are carnivorous marine gastropod molluscs in the family Muricidae, commonly called "murexes" or "rock snails".Houart, R.; Gofas, S. (2010). Murex Linnaeus, 1 ...

es, periwinkles,

volute A volute is a spiral, scroll-like ornament that forms the basis of the Ionic order, found in the capital of the Ionic column. It was later incorporated into Corinthian order and Composite column capitals. Four are normally to be found on an ...

s and

whelk Whelks are any of several carnivorous sea snail species with a swirling, tapered shell. Many are eaten by humans, such as the common whelk of the North Atlantic. Most whelks belong to the family Buccinidae and are known as "true whelks." Othe ...

s, as well as numerous freshwater groups, and some land snails with an operculum.

Description

The majority of prosobranchs have an operculum, a

corneous Corneous is a biological and medical term meaning horny, in other words made out of a substance similar to that of horns and hooves in some mammals. The word is generally used to describe natural or pathological anatomical structures made o ...

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

plate situated on the

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage The fus ...

surface of the foot. In many prosobranchs, the animal can completely close 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 ...

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 and Pulmonata. The subclass Prosobranchia (

Henri Milne-Edwards Henri Milne-Edwards (23 October 1800 – 29 July 1885) was a 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 Frenchwoman. Hen ...

) was further divided into the orders Archaeogastropoda, Mesogastropoda and Neogastropoda. 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, Education center, education and Research institute, research centers, created by the Federal government of the United States, U.S. government "for the increase a ...

and

National Science Foundation The U.S. National Science Foundation (NSF) is an Independent agencies of the United States government#Examples of independent agencies, independent agency of the Federal government of the United States, United States federal government that su ...

, 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. * * * {{Authority control Obsolete gastropod taxa Gastropod taxonomy Mollusc subclasses