Not to be confused with the bivalve order Protobranchia.
Prosobranchia was a large taxonomic subclass

Class (biology)

In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of sea snails, land snails and freshwater snails. This taxon of gastropods dates back to the 1920s. It has however been proven to be polyphyletic

Polyphyly

A polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...

 (consisting of more than one lineage of descent), and therefore it is no longer acceptable to be used as a taxon, because taxonomy

Taxonomy

Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 must reflect phylogeny, in other words the classification of a group must reflect its evolutionary descent, as far as that is known.

One can still encounter this subclass used as if still valid in many texts and websites. When Prosobranchia is not accepted as a taxon, still the term prosobranch can be legitimately used as an anatomically-descriptive adjective or noun.

Prosobranch means gill

Gill

A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s in front (of the heart

Heart

The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

). In contrast opisthobranch

Opisthobranchia

Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping...

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

Mantle (mollusc)

The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

 cavity and anus

Anus

The anus 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, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

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

The majority of marine

Marine (ocean)

Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...

 gastropods

Gastropoda

The Gastropoda or gastropods, more commonly known as snails and slugs, are a large taxonomic class within the phylum Mollusca. The class Gastropoda includes snails and slugs of all kinds and all sizes from microscopic to quite large...

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

Conch

A conch is a common name which is applied to a number of different species of medium-sized to large sea snails or their shells, generally those which are large and have a high spire and a siphonal canal....

es, cones

Conus

Conus is a large genus of small to large predatory sea snails, marine gastropod molluscs, with the common names of cone snails, cone shells or cones. This genus is placed in the subfamily Coninae within the family Conidae. Geologically speaking, the genus is known from the Eocene to the Recent ...

, cowries

Cowry

Cowry, also sometimes spelled cowrie, plural cowries, is the common name for a group of small to large sea snails, marine gastropod molluscs in the family Cypraeidae, the cowries...

, limpet

Limpet

Limpet is a common name for a number of different kinds of saltwater and freshwater snails ; it is applied to those snails that have a simple shell which is more or less conical in shape, and either is not spirally coiled, or appears not to be coiled in the adult snails.The name limpet is most...

s, murex

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 calle "murexes" or "rock snails"...

es, periwinkles, volute

Volutidae

Volutidae, common name volutes, are a taxonomic family of predatory sea snails that range in size from 9 mm to over 500 mm, marine gastropod mollusks...

s and whelk

Whelk

Whelk, also spelled welk or even "wilks", is a common name used to mean one or more kinds of sea snail. The species, genera and families referred to using this common name vary a great deal from one geographic area to another...

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

Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

.

Description

The majority of prosobranchs have an operculum

Operculum (gastropod)

The operculum, meaning little lid, is a corneous or calcareous anatomical structure which exists in many groups of sea snails and freshwater snails, and also in a few groups of land snails...

, a corneous

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....

 or calcareous

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:...

 plate situated on the dorsal

Dorsum (biology)

In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

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

Aperture (mollusc)

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

 with the operculum.

The nervous system of prosobranchs is twisted into a figure 8 due to a developmental process known as torsion

Torsion (gastropod)

Torsion is an anatomical event which takes place during the very early part of the life of snails and slugs of all kinds. In other words, torsion is a gastropod synapomorphy which occurs in all gastropods during larval development. Torsion is the rotation of the visceral mass, mantle and shell 180˚...

. The eyes are situated at the base of the tentacle

Tentacle

A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s.

Taxonomic context

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

Johannes Thiele

Johannes Thiele, full name Karl Hermann Johannes Thiele was a German zoologist specialized in malacology. His Handbuch der systematischen Weichtierkunde is a standard work...

) divided Gastropoda into three subclasses: Prosobranchia, Opisthobranchia

Opisthobranchia

Opisthobranchs are a large and diverse group of specialized complex marine gastropods previously united under Opisthobranchia within the Heterobranchia, but no longer considered to represent a monophyletic grouping...

 and Pulmonata

Pulmonata

The Pulmonata, or "pulmonates", are an informal group 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 subclass Prosobranchia (Henri Milne-Edwards

Henri Milne-Edwards

Henri Milne-Edwards was an eminent French zoologist.Henri Milne-Edwards was the 27th child of William Edwards, an English planter and militia colonel in Jamaica and Elisabeth Vaux, a French. He was born in Bruges, Belgium, where his parents had retired. At that time, Bruges was a part of the...

) was further divided into the orders Archaeogastropoda

Archaeogastropoda

Archaeogastropoda 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 having two gills and a double-chambered heart, with the eggs and sperm discharged...

, Mesogastropoda

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....

 and Neogastropoda

Neogastropoda

Neogastropoda is an unranked taxonomic clade of sea snails, marine gastropod mollusks. For many years prior to 2005 Neogastropoda was an order.- Description :...

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

Taxonomy of the Gastropoda (Ponder & Lindberg, 1997)

The taxonomy of the Gastropoda, as revised by Winston Ponder and David R. Lindberg in 1997, is an older taxonomy of the class Gastropoda, the class of molluscs consisting of all snails and slugs...

.