Gastropoda


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Related to Gastropoda: Echinodermata, class Gastropoda, Brachiopoda

Gastropoda

The largest and most varied class in the phylum Mollusca, possibly numbering over 74,000 species and commonly known as snails.

General characteristics

The shell is in one piece which, in the majority of forms, grows along a turbinate (equiangular) spiral (see illustration), but which is modified into an open cone in various limpets or is secondarily lost in various slugs.

Longitudinal section ground through the shell of a specimen of Conus spurius to reveal the central columella and spiral of whorls expanding to the apertureenlarge picture
Longitudinal section ground through the shell of a specimen of Conus spurius to reveal the central columella and spiral of whorls expanding to the aperture

All gastropods, at some time in their phylogeny and at some stage in their development, have undergone torsion. The process does not occur in any other mollusks. It implies that the visceral mass and the mantle shell covering it have become twisted through 180° in relation to the head and foot. As a result of torsion, all internal organs are twisted into a loop. Similarly in gastropods, the mantle cavity (the semi-internal space enclosed by the pallium or mantle) containing the characteristic molluscan gills (ctenidia) has become anterior and placed immediately above and behind the head. The most primitive gastropods retain a pair of aspidobranch (bipectinate or featherlike) gills, each with alternating ctenidial leaflets on either side of a ctenidial axis in which run afferent and efferent blood vessels. Lateral cilia on the faces of the leaflets create a respiratory water current (toward the midline and anteriorly) in the direction opposite to the flow of blood through the gills, to create the physiological efficiency of a countercurrent exchange system.

Classification and diversity

The usual systematic arrangement of the class Gastropoda involves three somewhat unequal subclasses. The first, the largest and most diverse, is the subclass Prosobranchia, which is made up largely of marine snails all retaining internal evidence of torsion. Prosobranchs are divided into at least four orders: Archaegastropoda, Caenogastropoda, Neritida, and Patellogastropoda; three superfamilies remain to be assigned to one of the four orders, and may each comprise a distinct order. The other two subclasses (Opisthobranchia and Pulmonata) are each considerably more uniform than the subclass Prosobranchia and, in both, the effects of torsion are reduced or obscured by secondary processes of development and growth.

More than half of all molluscan species are gastropods, and they encompass a range from marine zygobranchs, which can be numbered among the most primitive of all living mollusks, to the highly evolved terrestrial air-breathing slugs and snails. Pulmonates and certain mesogastropod families are the only successful molluscan colonizers of land and fresh waters.

Fossils

Fossil gastropods have a long geologic history, being common throughout the Paleozoic and increasingly abundant in the Mesozoic and Cenozoic. All three subclasses are known in the fossil record; many superfamilies, particularly prosobranchs, are extinct. Average duration of a genus has been estimated to range from 30,000,000 to 90,000,000 years.

Marine gastropods are important stratigraphic indicators in Cenozoic strata and locally are abundant in Cretaceous rocks. They are less common and less useful in the Jurassic and Triassic. Although individual genera have stratigraphic utility within the Paleozoic, it is only in the Ordovician that they are significant for correlation. See Mollusca

Gastropoda

[ga′sträp·ə·də]
(invertebrate zoology)
A large, morphologically diverse class of the phylum Mollusca, containing the snails, slugs, limpets, and conchs.
References in periodicals archive ?
In contrast, adults ate many Gastropoda and larval Diptera, with Collembola, Amphipoda, and other invertebrates present in lower numbers.
Bakkhali River Meghna River Estuary Estuary Benthos Groups S-1B S-2B S-3B S-4M S-5M S-6M Polychaete 1555 2089 1733 711 1733 1289 Oligochaete 221 266 177 267 311 222 Arthropods 266 445 311 311 533 489 Bivalvia 311 400 311 0 0 0 Gastropoda 1066 933 711 0 0 0 Unidentified 400 355 177 266 355 221 Total 3819 4488 3420 1555 2932 2221 Mean [+ or -] SD 3909 [+ or -] 540 2236 [+ or -] 689 Benthos Groups Mean [+ or -] SD Total Percentage (%) Polychaete 1518.
Gastropoda larvae (53-500 mm) was appeared in the ponds at 3 28 days of culture period and exists till end of culture period in aged ponds, whereas, it was not significant in new ponds.
The larger size class generally had more terrestrial prey taxa such as Hymenoptera, Gastropoda, Diplopoda and Coleoptera, although the latter three taxa were too rare to be incorporated into our statistical analysis.
A new genus of Ancillinae (Mollusca, Gastropoda, Olividae) from New Caledonia, with description of two new species.
El genero Bulimus Leach, 1814, (Mollusca, Gastropoda, Stylommatophora) en la Republica Argentina.
The ecology and physiology of these organisms have attracted great interest as they represent the transition between marine and land mollusks; they are probably the most primitive living group of Pulmonata and thus their study is key to understanding the evolution of the Gastropoda (Morton, 1955a, b; Martins, 1996a, b, 2001).
Gastropoda (378; 8%) and Coleoptera (339; 7%) were a distant third and fourth and other taxa were less common [ILLUSTRATION FOR FIGURE 1 OMITTED].
Third, both species are representatives of a taxon with an uncertain phyletic position, somewhere between the lower Gastropoda and the bifurcation of the gastropod tree into the caenogastropod and heterobranch branches (Haszprunar 1988; Ponder and Lindberg 1995).
Alternatively, it is possible that the morphological features that are used to classify ect-and ent-aquasperm in the Gastropoda need some reconsideration.