Prosobranchia
|
|
Prosobranch signifies "gills in front of the heart", this in contrast with opisthobranch: "gills to the right and behind the heart"
The majority of the gastropods (marine snails, land snails) are prosobranch. They have their gill, mantle cavity and anus situated in front of the heart (in contrast with the opisthobranchs). Most prosobranchs have different sexes and must cross-fertilize.
The prosobranchs are almost all marine snails, among them conchs, cones, cowries, limpets, murexes, periwinkles, volutes, whelks. They all have an operculum, a horny plate situated at the foot, by which the snail can close the aperture, when the body is retracted into the shell. Most have spirally coiled shells. Their nervous system has twisted into a figure-8 shape. The eyes are situated on tentacles.
The taxonomy of the gastropods, and their phylogenetic understanding has been evolving 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) was then further divided into the orders Archaeogastropoda, Mesogastropoda and Neogastropoda. Their new classification has been explained under their respective entry.
The Prosobranchia are now considered polyphyletic; i.e. containing more than one evolutionary line. Such a group is considered in error of classification. However, one can still encounter this old classification in many manuals (older and newer) and on most websites.
The term "prosobranch" can still be used in a descriptive way, meaning "a gastropod with the gill in front of the heart".
Reference
- 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 cladistic analysis. Replaces both the Archaeogastropoda with a number of distinct orders, and rejects the old three-fold division of Prosobranchia, Opisthobrancha, and Pulmonata.