Gastropoda

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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

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.

Gastropoda

[ga′sträp·ə·də]
(invertebrate zoology)
A large, morphologically diverse class of the phylum Mollusca, containing the snails, slugs, limpets, and conchs.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
9) clarified the varied usage and ambiguity of sinus, slit and selenizone in connection with the interpreted exhalant zone in Palaeozoic gastropods. He applied the term 'peripheral band' to the trace at the acute angulation on the upper whorl surface in Asgardaspira (Fig.
Encapsulated veligers of many gastropods retain prototroch, metatroch, and food groove, with which they feed on particles from nurse eggs; and an evolutionary return to planktotrophy is inferred for a calyptraeid whose ancestor lacked a planktotrophic stage but fed with opposed prototroch and metatroch during its encapsulated development (Collin et al., 2007).
In Ukraine, the northern Polissya regions, geographical center of which is in the territory of Kyiv and Zhytomyr regions, are characterized by the largest species diversity of gastropods in the small rivers of the Dnipro basin (Zhytova and Kornyushyn, 2017).
The ecology, age and growth of two muricacean gastropods from shores of Kuwait.
Can naticid gastropod predators be identified by the holes they drill?
tenuis infection in gastropods is positively correlated with increased deer density has not been adequately tested.
Of the seven molluscan classes, gastropods make up more than 80% of the species and majority of gastropod species exhibit an extremely limited mobility or are completely sessile as adults.
The fissures and cracks in these rocky shores contain water during low tide and so some gastropods tolerate survival in such fissures (Fig.
At the top of the list of usual suggestions for preventing deer worm infection in goats and sheep is to control both white-tail deer and gastropods. That's pretty much like asking you to herd cats.
Physa populations lived in synthetic spring water (US EPA, 2002) in aquaria (gastropod density <1.5 individuals/liter) at 22 C 3 C with a 16:8 h light:dark photoperiod and received boiled spinach ad libitum, which decomposed and provided bacteria for gastropods to graze on, similar to periphyton (Warner, 1976).
Lead concentrations in bivalves, crustaceans, gastropods, and cephalopods in five sampled months were in the ranges of 0.051-0.114, 0.01-0.064, 0.04-0.103, and 0.007-0.009 mg/kg, respectively.