Prosobranchia

Prosobranchia used to be a large taxonomic subclass of snails (gastropod mollusks). However, that version of gastropod taxonomy dates back to the 1920s, and is now considered to be outdated because that subclass has been proven to be polyphyletic; i.e. the group contains more than one evolutionary line, and therefore is no longer acceptable as a way of classifying the Gastropoda. However, one can still encounter this subclass in many texts (both older and newer), and on many websites.

Although taxonomically outdated, the term "prosobranch" can still be legitimately used in a descriptive way, as an anatomical term applied to certain kinds of gastropods. It is a reference to their anatomy and development, and it means "gills in front of the heart". (This is in contrast to the term opisthobranch, which means "gills to the right and behind the heart.")

The majority of marine gastropods are prosobranch, as are a few land snails and freshwater snails. In other words, these snails have their gill, mantle cavity and anus situated in front of the heart (in contrast with the arrangement in the opisthobranchs). Most prosobranchs also have separate sexes. The prosobranch gastropods include the majority of marine snails, among them conches, cones, cowries, limpets, 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 horny plate situated on the dorsal surface of the foot. In many, but by no means all prosobranchs, the snail can completely close the aperture with the operculum when the body is retracted into the shell. Most have spirally coiled shells, as is true of most gastropods in general.

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

Taxonomic context

The taxonomy of the gastropods and their phylogenetic understanding has been changing rapidly in the last few years. The old classification (J. Thiele 1929-1935), with the class Gastropoda divided into three subclasses Prosobranchia, Opisthobranchia and Pulmonata, is no longer accepted. The subclass Prosobranchia (Milne Edwards, 1848) used to be further divided into the orders Archaeogastropoda, Mesogastropoda and Neogastropoda. The new version of the gastropod classification is explained under those respective entries.

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 [Polyplacophora] ; Gastropoda: Prosobranchia)". Smithsonian Institution and National Science Foundation, 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.
*Bieler, R., 1992. "Gastropod phylogeny and systematics". "Annual Review of Ecology and Systematics", 23: 311-338.
*Haszprunar, G. 1988. "On the origin and evolution of major gastropod groups, with special reference to the Streptoneura". "Journal of Molluscan Studies", 54: 367-441. - an important paper on gastropod systematics using "clado-evolutionary" analysis. Replaces both the Archaeogastropoda with a number of distinct orders, and rejects the old three-fold division of Prosobranchia, Opisthobrancha, and Pulmonata. See also the following entry.
* Bieler, R., 1990. Haszprunar's "clado-evolutionary" classification of the Gastropoda - a critique. "Malacologia", 31(2): 371-380